Download Sookie, a student in Genetics 200A, is a little too obsessed with

Sookie, a student in Genetics 200A, is a little too obsessed with vampire TV shows. She keeps a mirror on
her bench that she uses to see if any of her mutants fail to show a reflection (a telltale sign of yeast
vampirism). Sookie recently obtained a number of wild S. pombe strains that were found growing outside
the nuclear reactor sites around the world. Interesting in the question of how heterochromatin is kept
within its normal boundaries, she inserted into each these strains a ura4 reporter gene just outside the
boundaries of the mat2/3 silent mating type locus in each of her wild S. pombe isolates. After crossing
two strains found outside of different reactors, she was astonished to see that an occasional progeny that
did not produce a reflection. She also replica-plates the colonies to 5-FOA. The plates below show the
patterns observed.
1. What types of tetrads are observed and in what ratios?
PD: 9
NPD: 1 (tetrad #1)
TT: 2 (Tetrad #7 and #12)
2. What does this tell you about the location of the mutations that are segregating in this genetic
cross?
You need both mutations to see a phenotype, and these two mutations are closely linked because PD >>
tetratype or NPD
3. What does this tell you about how genotype is connected to phenotype?
Mutations in both genes are required for vampirism and ura4 silencing.
Assume Strain 1 is Ab and strain 2 is aB:
Parental Ditype
Non-parental ditype
Ab
Ura4+, wt
AB
Ura4+, wt
Ab
Ura4+, wt
AB
Ura4+, wt
aB
Ura4+, wt
ab
Ura4-, vampire
aB
Ura4+, wt
ab
Ura4-, vampire
Tetratype
AB
Ab
aB
ab
Ura4+
Ura4+
Ura4+
Ura4-, vampire
4. Assuming that silencing of a ura4+ gene that is placed outside of heterochromatin means that
there has been a lateral spread of heterochromatin outside of its normal boundaries, suggest a
hypothesis (or two) for what might be the molecular cause of fission yeast vampirism.
Many possible correct answers, in general, yeast vampirism is caused by the spread of silencing outside of its
boundaries, or involves silencing of a specific gene outside of heterochromatin that is now silenced. Ie,
mutations in two genes necessary to cause ectopic spread of heterochromatin or a mutation in one gene that
causes spread of heterochromatin, which then silences a gene (vamp) required for vampirism.
A few examples explanations:
1) Mutations in the sequences that define the heterochromatic region at the mating locus (ie IRR or IRL
sequence, resulting in silencing of the gene that causes vampirism, near the mat2/3 locus.
2) Mutations in two separate components of a large protein complex that is involved in preventing
heterochromatin from spreading. A single mutation is insufficient to cause a phenotype, but two mutations in
the same complex make the protein complex non-functional.
3) A gain of function mutation in multiple proteins that normally result in gene silencing (swi6, clr4, etc)
4) Two genes, A and B encode for redundant silencing factors that when absent allow heterochromatin to spread.
Possibly my favorite explanation:
“Assume that vampirism is caused by mutations in the drac1 and ula1 loci. The fact that ura4+ is dependent on the
appearance of the vampire phenotype imply that in order drac1+ and ula1+ function in parallel to inhibit spread of
heterochromatin. Having either wildtype protein is sufficient to prevent spread of heterochromatin outside the mat2/3
locus. How does this also cause yeast vampirism? Perhaps a third gene pathway called vamp-1 is located in a usually
silenced region. Expression of this gene inhibits vampirism, perhaps by making enough yeast blood to stave off its
ravenous thirst. When heterochromatin spread silences the vamp-1 gene, the yeast can no longer produce yeast-blood.
Another possibility is that the spread of heterochromatin is sensed by a transcription factor, bleh1, with its
chromodomain. When bleh1 binds heterochromatin it dimerizes and becomes competent to bind DNA eventually
activating a host of vampire related genes (fang-growth, negative phototaxis, sparklyness, etc). Because there is always
some amount of heterochromatin present in the genome, this implies that there is some threshold level of dimerized
bleh1 below which is it unable to activate transcription strongly enough to undergo vampirization.”
Vampirism is caused by two genes, which together, also cause heterochromatin spreading.