Download Spatially ordered transcription of regulatory DNA in

321
Development 107, 321-329 (1989)
Printed in Great Britain © The Company of Biologists Limited 1989
Spatially ordered transcription of regulatory DNA in the bithorax complex of
Drosophila
ERNESTO SANCHEZ-HERRERO and MICHAEL AKAM
Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3EH, UK
Summary
The identities of the second through ninth abdominal
segments of Drosophila are specified by two genes of the
bithorax complex (BX-C), abdominal-A (abd-A) and
Abdominal B (Abd-B). The correct deployment of these
two genes requires an extensive region (the iab region)
located between the two protein-coding transcription
units. We show here that one iab mutation affects the
pattern of expression of Abd-B. We also show that most
or all of the DNA in this regulatory iab region is
transcribed. In blastoderm stage embryos we can define
three distinct domains within the iab DNA, each tran-
scribed in a region that extends from a characteristic
anterior limit to the posterior end of the segmented part
of the embryo. The anterior limits of expression for the
three regions are colinear with the sequence of the
domains on the chromosome, and lie at about twosegment intervals. We suggest that these early transcription patterns reflect the initial activation of the BX-C.
Introduction
Abd-B mutations (Lewis, 1978, 1981; Kuhn et al. 1981;
Sdnchez-Herrero et al. 1985; Karch et al. 1985). With
the exception of the iab-2 class, all the iab mutations
map between the abd-A and Abd-B transcription units
(Karch et al. 1985). All of these mutations fail to
complement abd-A and Abd-B mutations (Karch et al.
1985), therefore implying that they act in cis to alter the
expression of these genes (Karch et al. 1985; Casanova
et al. 1987; Peifer etal. 1987; Tiong etal. 1987; SdnchezHerrero et al. 1988). It is not known how the iab
elements specify unique parasegmental patterns of
expression for abd-A and Abd-B.
We show here by in situ hybridization to embryonic
tissue sections that the iab region is transcribed. The
patterns observed at blastoderm follow an anteroposterior order of expression and suggest an initial
double-parasegment subdivision for the activation of
the BX-C. We also demonstrate that an iab mutation
changes the distribution of Abd-B products.
The identities of some of the thoracic and all the
abdominal segments of Drosophila are specified by the
bithorax complex (BX-C) (Lewis, 1978). The BX-C
contains three genes, Ultrabithorax (Ubx), abdominalA (abd-A) and Abdominal-B (Abd-B), which in combination determine the characteristics of each segment
within the BX-C domain (Sdnchez-Herrero et al.
1985a,b; Tiong et al. 1985). Mutations in any of these
genes transform a series of parasegments (PS) (Martinez-Arias and Lawrence, 1985) which coincide with the
anatomical region where the products of the genes are
expressed (reviewed in Duncan, 1987).
The complex genetics of the BX-C can be simplified if
we classify BX-C mutations into two broad groups:
those that directly alter the products of the known
protein-coding transcription units, and those that affect
putative regulatory elements. For example, in the Ubx
domain, Ubx mutations eliminate or alter the expression of the Ubx protein throughout the embryo
(Beachy et al. 1985; Weinzierl et al. 1987), whereas the
regulatory mutations abx and bxd, which transform just
a subset of the parasegments affected in Ubx mutations
(Hayes et al. 1984; Casanova et al. 1985), change the
expression of Ubx in specific regions: abx in PS5 and
bxd in PS6-13 (White and Wilcox, 1985; Cabrera et al.
1985; Beachy et al. 1985).
Within the abdominal region of the BX-C, the infra
abdominal (iab) mutations each transform a subset of
the segments (or parasegments) affected by abd-A or
Key words: bithorax complex, transcriptional regulation,
infra abdominal transcripts.
Materials and methods
In situ hybridization
Whole lambda phages or gel-purified fragments were labelled
with 35S by the oligo-random-priming method (Feinberg and
Vogelstein, 1983) to specific activities of 4-19X108 disintsmin-'jug"'- l-l-5xl0 5 disintsmin~V~' were used. In situ
hybridization was as previously described (SSnchez-Herrero
and Crosby, 1988). The Abd-B probe used is a 2-3 kb Abd-B
cDNA (Hoey et al. 1986) that includes the homeobox. This
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E. Sdnchez-Herrero and M. Akam
10kb
40
30
60
50
SO
6004
70
2255
2226
90
100
2279
2253
abd-A
28 - 9.5%
37.5 - 11%
110
120
6053
130
140
150
8077
160
170
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190
—I
200kb
8087
8019
28 - 9.5%
20.5 - 9%
Abd-B
Fig. 1. Map of the abdominal region of the BX-C according to Karch et al. 1985, showing the probes that detect specific
transcription patterns at blastoderm (numbers above long probes correspond to lambda phages). Probes that detect each of
the three different iab patterns are distinctively shaded. Probes that hybridize to abd-A or Abd-B transcripts are shown by
black bars; those that detect no transcription at blastoderm are shown by white bars. The numbers below each iab region
indicate anterior and posterior limits of expression in percentage of egg length (EL; 0% corresponds to the posterior pole).
probe hybridizes to sequences that are common to all classes
of Abd-B transcripts (E.S. unpublished). The limits of
expression of the different probes at stages later than blastoderm were ascertained by hybridizing alternate sections with
Ubx, abd-A or Abd-B probes. Control experiments with wildtype lambda showed no specific signal at any stage of
development.
Mutations
The iab-7MX2 mutation (previously called Abd-BMX2) is a
chromosome break lying between +139-5 and 142 kb (Karch
et al. 1985). It transforms abdominal segments(A) 5 to 7 (or
PS 10 to 12) into segment A4 (or PS9) (Sanchez-Herrero et al.
1985; Duncan, 1987).
Results
We have analyzed transcription of the entire abdominal
region of the BX-C (+15 to +200 kb) (Karch et al. 1985)
by hybridizing probes prepared from genomic DNA
fragments to sections of Drosophila embryos. The
rationale was to identify previously unknown transcription units that might be active in only a few cells or
segments. To our surprise, we found that the majority
of tested fragments showed a distinct and spatially
restricted pattern of hybridization, although, in general, we did not find transcripts localized to specific
abdominal segments.
Transcription patterns at the blastoderm stage
At blastoderm, only fragments to the left of the abd-A
transcription unit (+15 to +30 kb) and those in the
distal end of the Abd-B transcription unit (+192 to
+ 199 kb) showed no signal (see also Kuziora and
McGinnis, 1988). At this stage, probes from +30 to
+58 kb showed the expected pattern of expression
corresponding to the abd-A gene (Rowe, 1987 and E. S.,
unpublished) and probes from +151 to +192 kb showed
patterns corrresponding to those of the Abd-B gene
(Harding and Levine, 1988; Sanchez-Herrero and
Crosby, 1988; DeLorenzi et al. 1988; Kuziora and
McGinnis, 1988). Probes from the iab region (+58 to
+ 150 kb) in between these genes showed one of three
distinct patterns. Between +58 and +86 kb (domain I),
all probes hybridized to a region of the blastoderm
extending from about 38% to 10% egg length. From
+99 to +125 kb (domain II), all probes hybridized to an
overlapping but more restricted region, approximately
from 28% to 10% egg length. Fragments from +125 to
+ 150 kb (domain III) hybridized only to the region
between around 20% and 10% egg length (Figs 1-3).
By hybridizing alternate sections with iab and fushi
tarazu (ftz) probes we confirmed that these three DNA
domains are transcribed in regions that correspond
approximately to the primordia for parasegments 9-15,
11-15 and 13-15, respectively (Martinez-Arias and
Lawrence, 1985).
Probes from domains I and II detect transcripts from
early cycle 14, just before the nuclei begin to elongate.
The hybridization signals observed with these probes
remain stronger than those obtained with comparable
abd-A and Abd-B probes until late cycle 14, when the
levels of abd-A and Abd-B transcripts increase substantially. Transcripts from domain III are first detected
somewhat later, in mid-cycle 14; hybridization signals
with domain III probes are never as strong as those
observed with comparable Abd-B probes on the same
embryos. Probes from the three domains detect signal
first and predominantly in the nuclei, although later
signal appears also over the cortical cytoplasm. Both
infra abdominal transcripts
40
30
20
10% egg length
2279
8004
\ ~w "vw
8019
8053
8060
8077
(38)
(16)
(2)
(12)
(22)
(6)
(8)
(18)
(41)
(2)
(10)
(9)
(6)
(4)
(10)
(2)
(20)
(12)
(6)
(2)
(16)
(8)
(4)
(2)
(14)
Fig. 2. Measures in percentage of egg length (EL) of the
different probes used. Each bar corresponds to a different
probe and the bars are shaded as in Fig. 1, according to the
group to which the probe belongs. The first two bars in
each group correspond to lamda phages (indicated by their
numbers) and the rest of the probes are ordered according
to their proximo-distal (left-right) order on the
chromosome (see Fig. 1). The numbers to the right indicate
the number of hemi-blastoderms studied. The limits of
transcription vary slightly along the dorso-ventral axis, but
no attempt was made to control for this. This variation will
account in part for the differences in measurements within
each group.
the ectoderm and mesoderm shows the same spatially
restricted expression of iab transcripts during gastrulation.
We obtained comparable signals with probes derived
from whole lambda clones and with some probes
derived from shorter subcloned fragments. In each case
several non-overlapping fragments within each domain
gave the same pattern of hybridization as the longer
probes (Fig. 1). This makes it unlikely that the transcripts are detected by virtue of spurious hybridization
to repeated sequences (No extensive repetitive sequences have been reported in the iab region, Karch et
al. 1985). At least in one case (+113—I-121kb, within
domain II), single-stranded probes from both strands
detect transcripts with the same spatial distribution,
implying the existence of at least two similarly regulated
promoters.
Transcription after blastoderm
The iab region continues to be transcribed throughout
embryogenesis, but the spatial patterns of expression
323
change as development proceeds (Fig. 4). Probes spanning the whole iab region (from +58 to +150 kb)
hybridize strongly to transcripts in PS13-15. When the
germ band is extended both ectoderm and mesoderm
show expression (Fig. 4C,E). Later, the signal is particularly strong in the ventral nerve cord, although the
ectoderm is also labelled (Fig. 4D,F). We can attribute
no function to these transcripts, as chromosome breaks
in the iab region have no apparent effect on the
development of PS13-15 (Karch et al. 1985). We do not
consider them further. Probes in the iab-3 and iab-4
regions detect a more complex pattern superimposed
on this basic one. Probes from +57 to +67kb show, in
addition to strong expression in PS13-15, weaker signal
in PS 8-12 (Fig. 4A). A probe extending from +79-5 to
+86 kb also detects, at the same stages, transcripts
anterior to PS13, extending to PS 8 or 9 (Fig. 4B). The
anterior limits of expression of these two patterns seem
to correlate with the most anterior parasegment affected by iab-3 and iab-4 mutations (Karch et al. 1985).
The intervening DNA (from +67 to +79-5 kb) shows
little or no hybridization in the region anterior to PS 13.
We have made no attempt to characterize in detail the
transcription units from which these transcripts arise.
The iab-7MX2 mutation affects the expression of Abd-B
products
To see if the iab region controls the expression of AbdB we have looked at the distribution of Abd-B transcripts in the mutation iab-7MX2. This mutation is an
inversion with a breakpoint at about +140 that separates the iab-3-iab-7 region from the Abd-B transcription unit (Fig. 5A) (Karch et al. 1985), and transforms
A5-A7 (or PS10-12) into A4 (or PS9; Fig. 5D) (SSnchez-Herrero etal. 1985; Duncan, 1987). We hybridized
an Abd-B cDNA (Hoey et al. 1986) or an Abd-B
genomic clone (the phage 8082, Karch et al. 1985) to
embryos from the stock iab-7MX2/TMl. In wild-type
embryos these two probes show the expression of all
Abd-B transcripts. Most of the embryos from the iab7MX2/TM1 stock showed the normal Abd-B pattern of
expression (Fig. 5C), extending from PS10 to PS15 in
late stages (Harding et al. 1985; Hoey et al. 1986;
Harding and Levine, 1988; Sanchez-Herrero and
Crosby, 1988; Kuziora and McGinnis, 1988). In about
one quarter of the germ-band-retracted embryos (23/
114) we observe that the expression of Abd-B is
restricted to PS13-15 (Fig. 5E) in the epidermis and
ventral cord. The parasegmental nature of this restricted expression is revealed in those sections that show
segmental grooves. The normal extension of Abd-B
expression into PS10-12 is therefore prevented by the
separation of regulatory sequences that control the
expression of Abd-B products. The control is over the
transcripts corresponding to the m element of Abd-B
(Casanova et al. 1986), since these are the only Abd-B
transcripts expressed in PS10-12 (Sanchez-Herrero and
Crosby, 1988; Kuziora and McGinnis, 1988). Alternate
sections for three of the embryos with restricted Abd-B
expression were hybridized with an abd-A probe. No
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E. Sdnchez-Herrero and M. Akam
Fig. 3. Patterns of expression at blastoderm of probes belonging to the three different iab regions. In this figure and in the
following ones the anterior part of the embryo is to the left. (A) Hybridization observed with a 6-5 kb EcoRl fragment from
+79-5 to +86kb, characteristic of domain I (+58kb to +86kb). (B) Pattern observed with probes of domain II (+99 to
+ 125kb); the probe is a 8kb Hindlll fragment from +113 to +121 kb. This section is alternate to the previous one. During
and after gastrulation probes from part of region II, +99 to +113, detect stronger signal in the posterior cells that express
this pattern. (C) Expression observed with a 6kb EcoRl fragment from +131-5 to+137-5kb; this is characteristic of probes
from +125 to + 150kb (domain III).
change was observed with respect to the normal abd-A
pattern.
20 blastoderm stage embryos from the same stock
were hybridized with a probe corresponding to iab
pattern II (the 8 kb Mndlll fragment used in wild-type
embryos, see Fig. 3). None of the embryos presented
any apparent change in the pattern of expression
compared to wild-type.
Discussion
The genetics of the BX-C suggests that the DNA of the
iab region controls the expression of abd-A and Abd-B
products. Genetic tests indicate that iab mutations act
essentially in cis. For example, the mutation iab-7MX2
when homozygous or in trans with a deficiency for AbdB transforms PS10-12 to PS9. We have shown that in
iab-7MX2 homozygous embryos Abd-B is not tran-
scribed in PS10-12. This observation supports the
hypothesis that the iab elements act at the DNA level as
distant regulatory elements to control abd-A and AbdB (Karch et al. 1985; Casanova et al. 1987; Peifer et al.
1987; Tiong et al. 1987; Sdnchez-Herrero et al. 1988).
Our results also reveal the unexpected finding that
the iab region is itself transcribed and shows restricted
patterns of expression. We discuss below the nature of
these transcripts, and their significance with respect to
the function of the BX-C.
Nature of iab transcripts
At present we do not know the number or structure of
iab transcription units. In parallel experiments to those
reported here, M. Crosby examined RNA from staged
embryos by filter hybridization, using probes from
domains II and III (+103—I-140 kb) and probes from
the Abd-B transcription unit. Abd-B transcripts were
readily detected (Sdnchez-Herrero and Crosby, 1988)
infra.ahdnminnl trnnscrintM
Fig. 4. Patterns of expression observed at stages of development later than blastoderm. Embryos oriented so that anterior is
to the left and dorsal is up. (A) Signal observed in an embryo with the germ band extended probed with a 1-8 kb EcoRl
fragment from +57 to +58-8 kb; signal is detected from PS 8 to 15, more strongly in PS13-15. (B) Expression detected with
a 6-5 kb EcoRl fragment from +79-5 to +86 kb. Signal in the ventral cord extends from PS8 or 9 to PS15, with stronger
expression in PS13-15. (C) Expression detected at the germ band extended stage by a 8 kb Hindlll fragment from +113 to
+ 121 kb; signal is restricted to PS13-15. (D) Signal observed at late stages of embryogenesis with a 5 kb EcoRl fragment
from +94-5 to +99-5kb. The strong expression in the ventral cord is also limited to PS13-15. (E) A 6kb EcoRl fragment
from +131-5 to +137-5 kb also detects expression just in PS13-15 when the germ band is extended. (F) When the germ band
is retracted the same fragment detects signal mainly in the ventral cord, also in PSD-15.
but, in the iab region, only weak smears of high
molecular weight RNA were seen. No discrete transcripts were sufficiently abundant to be reproducibly
detectable (M. Crosby, personal communication).
A possible clue as to the nature of these transcripts is
provided by the similarities that they show with the
bithoraxoid (bxd) transcripts of the Ultrabithorax
domain (Hogness et al. 1985; Lipshitz et al. 1987). Both
bxd (Akam et al. 1985; Irish et al. 1989) and iab
transcripts are detected early in cycle 14, before transcripts from the adjacent protein-coding genes; both
show a similar cellular distribution, first in the nuclei
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E. Sdnchez-Herrero and M. Akam
MX2
iab-7
"PSTO
PS 11
m
i
n
11
A1
6
7
8
m
i
A
(0
140
120
100
9
9
PS 12
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Abd-B
(m)
infra abdominal transcripts
Fig. 5. Correspondence between spatial regulation of AbdB products and phenotypic transformation of an iab-7
mutation. (A) Map of the distal part of the iab region with
the approximate location of the iab7MX2 mutation (Karch
etal. 1985). The hatching indicates the DNA region that is
separated from the promoter of the transcripts
correponding to the m element of Abd-B (see Kuziora and
McGinnis, 1988). (B) Abdomen of a wild-type male.
Numbers to the right represent parasegments. Abdominal
segments (A)l and 4 are indicated for reference. (C) AbdB expression in an embryo from the iab-7MX2/TMl stock
that has completed germ band retraction. The signal in the
ventral cord extends from PS10-15, as in wild-type
embryos, and we think that the genotype of this embryo is
iab-7Ux2lTMl or TM1/TMJ. (D) Abdomen of a male of
genotype iab-7MX2/DfP9. PS 10-12 are transformed into PS
9. We assume the adult transformation to be
parasegmental, like the embryonic one (Duncan, 1987).
(E) Abd-B expression in an embryo from the iab-7MX2j
TM1 stock. The embryo is in a similar orientation and stage
to that in C. The signal in the ventral cord is now restricted
to PS13-15, and we believe the genotype is iab-7MX2/iabJMX2 T n c cm t, r yos in C and E are from the same slide, so
all experimental conditions are the same.
and later also in the cortical cytoplasm. The bxd
transcripts derive from a long (25 kb) transcription unit,
and are inefficiently processed into a complex family of
variably spliced transcripts that appear not to code for
proteins (Lipshitz et al. 1987). We suspect that the iab
transcripts may have a similar organization.
The distribution of the iab transcripts does not
appear to be consistent with their having a role as
parasegment-specific products mediating iab functions.
We would expect such products to be expressed at peak
levels in those segments where they are required.
Although the regions of expression of the early iab
transcripts are in the same linear order as the effects of
iab mutations, only very low levels of transcript (if any)
are present in the most anterior segment affected by iab
mutations within each domain. Thus mutations that
affect PS10 and 11 (or A5 and A6) map within domain
II, but domain II transcripts can be detected at high
levels only from about PS11 backwards. A similar
'offset' is observed in the relationship between mutant
effect and transcript distribution in the bxd region of
Ubx (Akam et al. 1985; Irish et al. 1989).
The lack of a direct relationship between iab transcripts and function is supported by the observation that
the accumulation of domain II transcripts appears to be
normal in iab-7MX2 homozygous embryos. In this mutation domain II is intact (the chromosome break lies in
domain III, which is not detectably transcribed in
PS10-12). Thus the mutant phenotype in PS10-12 (and
the lack of expression of Abd-B in these parasegments)
is not likely to be caused by any disruption of domain II
transcripts per se. Rather, it is likely to be due to the
separation of PS10-12 regulatory sequences within
domain II (and most of domain III) from the Abd-B
transcription unit.
All of these arguments reinforce the conclusion
drawn from genetic data that the iab region serves to
327
regulate abd-A and Abd-B. In this context the iab
transcripts might function in cis by some unprecedented
mechanism. Alternatively, it may be the state of the iab
DNA, or the act of transcription, that mediates iab
function. The transcripts themselves might then be
functionless by-products of this mechanism (see Lipshitz et al. 1987 for a discussion of this point with respect
to bxd transcripts).
Transcription at blastoderm reveals early stages in the
activation of the BX-C
Although early iab transcripts may have no function,
they are significant because they reveal that, as early as
mid-cycle 14, DNA of the BX-C has sensed and
responded to a series of positional cues that subdivide
the presumptive abdomen. Four different regions are
defined by the transcription of 0, 1, 2 or 3 of the iab
domains, in progressively more posterior cells of the
blastoderm. This ordered activation of iab DNA occurs
shortly before the abd-A and Abd-B genes are first
transcribed (Rowe, 1988; Harding and Levine, 1988;
Sdnchez-Herrero and Crosby, 1988; DeLorenzi et al.
1988; Kuziora and McGinnis, 1988 and E.S., unpublished), and long before the differential expression of
the abd-A and Abd-B genes establishes differences
between the abdominal segments 3-7.
There is colinearity between the location of DNA
domains in the chromosome and the anterior limit to
which each is transcribed in the blastoderm: more
proximal domains are transcribed in more anterior
cells. This parallels the relationship between the position of each iab mutation on the DNA map and the
segments that it affects (Lewis, 1978): mutations that
map proximally affect anterior abdominal segments,
while those mapping distally transform more posterior
ones. We do not find a one-to-one correspondence
between expression domains and phenotypic classes of
mutations, but within our limits of resolution, each class
of iab mutations (Duncan, 1987) falls entirely within a
single transcriptional domain (Fig. 6): iab-3 and iab-4
mutations are included within domain I, iab-5 and iab-6
mutations are within domain II, and iab-7 mutations
within domain III.
Peifer et al. (1987) proposed that each class of iab
mutations identifies a parasegment-specific regulatory
domain within the BX-C. They suggested that these
were activated ('opened') in sequence along the
chromosome in response to cues along the anteroposterior axis of the embryo. We believe that blastoderm transcription in the iab region may be a reflection
of this activation event, as previously proposed for the
bxd transcripts in the Ubx region (Lipshitz et al. 1987).
If this is so, the initial domains of activation would
appear to define units approximately two parasegments
long, rather than individual parasegments, since the
anterior limits of expression of the iab transcripts are
separated by around 8-10% egg length.
The gap genes regulate the early activation of the
BX-C (White and Lehman, 1986; Ingham et al. 1986;
Harding and Levine, 1988; Irish et al. 1989) and are
expressed at maximal levels just as these iab transcripts
328
E. Sdnchez-Herrero and M. Akam
PS 8
PS 10
PS 14
PS 12
I
40
20
30
10% egg length
\^\\\\\\\N\\\\\\\\\\\s\\\\X\\\\\\\\\\\\N
n
in
m
H
30
50
70
iab-3
90
iab-4
iab-5
110
130
iab-6
iab-7
150
170
190kb
abd-A
Mcp
Fab
Abd-B
Fig. 6. The relationship between transcriptional activity in blastoderm stage embryos and chromosomal position for each of
the three iab domains. Above: The hatched bars at the top of the figure show the extent of the three blastoderm
transcription domains relative to egg length (posterior pole = 0 %), and to the approximate position of parasegment
primordia. Even-numbered parasegments are shaded on the scale bar. Below: The extent of chromosomal DNA showing
each of these three patterns is depicted by corresponding shaded bars above the DNA map of the abdominal region of the
BX-C. Shown below the map are the locations of abd-A and Abd-B transcription units (black bars), the spread of each class
of iab mutation (square brackets) and the extent of the small deletions Mcp and Fab (paired round brackets).
begin to accumulate (Knipple et al. 1985; Tautz et al.
1987). We would guess that they are directly involved in
the initial activation of regulatory domains and therefore in the establishment of the iab patterns of expression. Mutations in gap genes alter the limits of
expression of the iab transcripts (E.S., unpublished).
After this initial activation by gap genes, ftz and
perhaps other pair-rule genes may act to distinguish
within these domains regions that are unique to single
parasegments (Duncan, 1986; Ingham and MartinezArias, 1986).
The suggestion that the iab transcripts reflect the
initial activation of the BX-C accounts for the colinearity of transcription boundaries with mutant effects. If
the iab transcripts reflect a response to the gap genes
but not to the pair-rule inputs there would be no reason
to expect their limits of expression to be precisely
parasegmental. However, these assumptions provide
no obvious explanation for the apparent shift of about
one parasegment between high levels of expression of
iab transcripts from each domain, and the structures
affected by mutations within the same domain. This
discrepancy may disappear once the iab promoters are
precisely located with respect to the DNA elements that
sense position at blastoderm.
This model is similar to that previously proposed by
Duncan (1986), in which the initial activation of the BXC is made in double-parasegmental units (protosegments), subsequently divided by ftz. Both of these
models imply two different processes required for the
correct deployment of BX-C genes. Initially, gap and
pair-rule genes define regions in the BX-C regulatory
DNA that correspond to individual parasegments (the
iab transcripts perhaps being a reflection of the initial
activation). Thereafter, these regulatory regions with
parasegmental 'addresses' regulate the expression of
abd-A and Abd-B products in response to the evolving
pattern of positional cues within each segment, so that
segmental identity is achieved.
The location of two mutations, Miscadastral pigmentation (Mcp) and Frontabdominal (Fab), is of particular
interest in relation to the delimitation of parasegmental
domains in the iab DNA. These two mutations are short
deletions (Karch et al. 1985; F. Karch, personal communication) yet they result in dominant phenotypes
(Lewis, 1978; F. Karch, personal communication), Mcp
activating the iab-5 regulatory domain in A4 and Fab
activating the iab-7 regulatory domain in A6. It has
been suggested (F. Karch personal communication)
that they may act by removing the boundaries between
two parasegmental regulatory domains. We note that
they lie at or very close to the boundaries between
transcription domains I and II (for Mcp) and II and III
(for Fab) (Fig. 6).
We thank W. Bender and S. Sakonju for providing phages,
plasmids and DNA, M. Crosby and F. Karch for communicating unpublished results, R. Weinzierl, A. Rowe and I.
Dawson for help, M. Levine for the Abd-B cDNA, A.
Martinez-Arias for the ftz probe, E. Reoyo for technical
assistance and M. Crosby, G. Morata, S. Sakonju and J.
Casanova for comments on the manuscript. E.S. is on leave
from the CSIC, Spain. This work has been supported by
EMBO and by the Medical Research Council.
References
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infra abdominal transcripts
(1985). Function and expression of Ultrabithorax in the
Drosophila embryo. Cold Spring Harbor Symp. quant. Biol. 50,
195-200.
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