Download The Influence of Ammonium Permease Activity and

Journal of General Microbiology (1987), 133, 375-379. Printed in Great Britain
375
The Influence of Ammonium Permease Activity and Carbon Source
on the Uptake of Ammonium from Simple Defined Media by
Saccharomyces cevevisiae
By E M M A N U E L E. EGBOSIMBA A N D J . C O L I N S L A U G H T E R *
Department of Brewing and Biological Sciences, Heriot- Watt University, Chambers Street,
Edinburgh E H l I H X , UK
(Received 19 May 1986; revised 9 July 1986)
When growing under defined conditions, cells of the yeast Saccharomyces cerevisiae absorbed
ammonium more rapidly with glucose as carbon source than with maltose. Ammonium pool
sizes and permease activity were also higher in the glucose-grown cells and the relationship
implies that firstly, the sugar is a primary determinant of ammonium permease activity and,
secondly, the activity of the permease largely determines both the rate of ammonium uptake and
ammonium pool size in the first part of the fermentation.
INTRODUCTION
The plasma membrane of yeast cells is permeable to free ammonia in solution but at the pH of
most yeast fermentations the compound is present almost entirely as the ammonium ion and this
form requires an active uptake process (Roon et al., 1975). Ammonium permease has been
shown to be separate from the amino acid permeases (Roon et al., 1977a) and the mutually
inhibitory effects found in both transport systems have been explained by assuming competition
for a common energy source rather than through an affinity of the inhibiting molecule for the
other transport system (Roon et al., 1977b). In simple defined media, ammonium may be the
only source of nitrogen for synthesis of amino acids, proteins and nucleic acids. It is used very
efficiently by Saccharomyces cerevisiae and, as a sole source of nitrogen, is only bettered by
asparagine, aspartic acid and glutamine (Thorne, 1945; Dubois et al., 1974), but there are a
number of reports that ammonium may have other significances as well. The studies of Yoshino
& Murakami (1982) with permeabilized yeast showed that ammonium could activate
phosphofructokinase and pyruvate kinase and they suggested a role for ammonium control of
glycolysis. On the basis of fermentation studies, Saita & Slaughter (1984) felt that the action of
ammonium on glycolysis was beter understood as a secondary effect arising from stimulation of
protein synthesis by ammonium in its capacity as a nitrogen source. However, in experiments
with intact cells of S. carlsbergensis, Lloyd et al. (1983) showed that addition of ammonium
resulted in a short-lived increase in C 0 2 output over a 5-10 min period, which could well have
been due to direct activation of the glycolytic enzymes by ammonium. Another unexpected
effect of ammonium was noted by Slaughter & Housden (1978), who reported that high levels of
ammonium in excess of that required for growth caused a decrease in the formation of higher
alcohols but a slight increase in the production of ethanol. The mechanism of this effect remains
unknown, but Devine & Slaughter (1980) found that it occurred only in the presence of glucose
and not with a range of other fermentable sugars.
Assimilation of ammonium within the cell into glutamate and other amino acids has been well
studied in yeast (Brown, 1980) but little is known about the control of ammonium uptake. This
paper describes experiments carried out to assess the importance of the permease system in
control of ammonium uptake by S. cerevisiae during fermentation of defined sugar/salts media.
0001-3461 0 1987 SGM
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Fri, 16 Jun 2017 21:22:16
376
E. E . EGBOSIMBA A N D J . C. SLAUGHTER
METHODS
Yeast maintenance andpropagation. Saccharomyces cerevisiae NCYC 1108 was maintained on malt agar slopes at
4 "C. As required, yeast was pre-cultured in a malt extract medium for 3 d at 25 "C, harvested by centrifugation,
washed twice with chilled distilled water and finally resuspended in chilled water at approximately 3 x lo9 cells
ml-' prior to inoculation into the experimental media.
Experimentalfermentation. The composition of the defined medium was described by Harding et al. (1984). All
media contained ammonium sulphate at 1.18 g1-1 and either glucose at 8Og1-I or maltose at 76gl-'.
Fermentations were carried out at 20 "C in 500 ml conical flasks containing 250 ml medium and sealed with a
fermentation lock. The flasks were incubated either without disturbance or on an orbital shaker at 130 r.p.m. The
initial cell count was approximately 6 x lo6 ml-1 as determined using the improved Neubauer haemocytometer.
In all the experiments described in this paper fermentations were carried out in duplicate and sampling for each
determination was also in duplicate. The results shown are the means.
Determinationofsugar and ammonium concentration.The concentrations of glucose and maltose were determined
in samples from which the cells had been removed by centrifugation, using the reducing sugar determination of
Somogyi & Nelson (1944). The ammonium concentration of the samples was determined using the urea/ammonia
kit supplied by Boehringer Mannheim.
Determinationof ammonium pool size. A sample of fermenting medium was passed through a Nuflow membrane
filter (0.45pm pore size) and the retained yeast washed twice with 10 ml portions of chilled water. The filter was
then transferred to a tube containing boiling water and boiling was continued for a further 20 min. The aqueous
extract was then cooled, clarified by centrifugation and made up to the original sample volume with distilled
water before assay of the ammonium content using the technique described in the preceding section. Preliminary
experiments showed that if the cells were washed only once then the results were variable. Two washes gave
consistent results and the values were unaffected by a third wash.
Determinationof ammonium permease actiuity. The [ 14C]methylammonium analogue assay described by Roon et
al. (1975) was used with 10 mM-sodium citrate buffer, pH 6.5, and a cell density of 2.61 x lo7 cells ml-I. The sugar
used in the assay was the same as that present in the fermentation medium.
RESULTS
Influence of culture conditions on the uptake of sugar and ammonium. The fermentation of the
defined medium containing either glucose or maltose as carbon source was followed under nonshaken and shaken conditions. Whereas there was a greater increase in cell number in the
shaken cultures, the rate of uptake of ammonium from a particular medium was more or less
unaffected (Fig. 1). In contrast, shaking the flasks caused a slight reduction in the rate of uptake
of maltose and a distinct reduction in the rate of glucose uptake. Under both shaken and nonshaken conditions the rate of uptake of ammonium was greater in the presence of glucose as
compared to maltose.
Influence of carbon source on the intraceflufar ammonium concentration. Measurement of
ammonium pool sizes during fermentation under shaken conditions (Fig. 2a) showed a distinct
peak at 5-10 h in the presence of either glucose or maltose but the pool size in the presence of
glucose was persistently higher than in the presence of maltose.
Influence of carbon source on the ammoniumpermease activity of the cells. As was the case for the
ammonium pools, ammonium permease showed a peak during fermentation of either glucose or
maltose in shaken cultures (Fig. 2). The peak in permease activity appeared to occur just after
the peak in pool size and once again, the activity in the presence of glucose was consistently
higher than in the presence of maltose.
The uptake activities found ranged between 17-3 nmol methylamine ml-l min-l for cells
from the glucose medium after 10 h and 2.9 nmol ml-* min-' for cells from the maltose medium
after 24 h. The variation between duplicates was normally less than 5 % and in no case greater
than 10%.
DISCUSSION
Initial experiments (Fig. 1) showed that shaking appeared to allow a more efficient use of the
sugars, particularly glucose, in terms of cell production. However, the initial rate of ammonium
uptake was unaffected by shaking but was dependent on the carbon source under both the
fermentation conditions tested, with a higher rate of uptake occurring in the presence of glucose.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Fri, 16 Jun 2017 21:22:16
377
NHX permease control of NHI uptake by
r
z
h
€
W
C
E
10
5c
;
.-
3 t
30
Time (h)
OO
,
,
30
Time (h)
t
L1 0
OO
30
Time (h)
L
OO
30
Time (h)
Fig. 1. Effect of fermentation conditions on ammonium uptake (see Methods for experimental
details). (a) Shaken culture. 0,
Glucose concentration; 0 ,maltose concentration (as hexose); , cell
number in glucose medium; I,
cell number in maltose medium. (6) Shaken culture. 0,
ammonium
concentration in glucose medium; 0,
ammonium concentration in maltose medium. (c) Non-shaken
culture. Symbols as in (a). (d)Non-shaken culture. Symbols as in (b).
C
I
Time (h)
Time (h)
Fig. 2. (a) Effect of sugar on the internal concentration of ammonium and ammonium uptake ability.
See Methods for experimental details. 0 , 0,
Concentration of ammonium in cells from glucose
, I,
Ammonium uptake ability of cells from glucose
medium ( 0 )and from maltose medium
medium ( 0 )and from maltose medium (I).
(b) Effect of sugar on the relative changes in ammonium
permease activity and internal ammonium pool during fermentation. 0 , Ratio of the ammonium
permease activity in cells from glucose as against maltose media; 0 ,ratio of the ammonium pool size in
cells from glucose as against maltose media.
(a).
This observation was reinforced by measurement of the ammonium pool size, which was also
higher when glucose was being fermented as compared with maltose (Fig. 2a). As the amount of
growth was not affected by the carbon source, although the uptake rate was, it seems unlikely
that different rates of assimilation of ammonium into amino acids could account for differences
in pool size. This supposition is supported by earlier measurements made on cells cultivated in
the same media under non-shaken conditions (Saita, 1983), which showed that NADPDownloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Fri, 16 Jun 2017 21:22:16
378
E. E. EGBOSIMBA AND J . C. S L A U G H T E R
Time (h)
Fig. 3. Effect of sugar on the observed and calculated rates of ammonium uptake during fermentation.
0,
Observed rates in glucose medium (from Fig. 1 b); 0 ,calculated rates in glucose medium (from cell
numbers in Fig. l a and specific activity in Fig. 2). 0,
Observed rates in maltose medium (as for
glucose); W , calculated rates in maltose medium (as for glucose).
dependent glutamate dehydrogenase activity was not affected by carbon source and,
furthermore, was at an approximately tenfold excess as compared with the observed rate of
ammonium uptake.
These data implied that the permease activity itself could be important in limiting uptake and
in determining pool size. Fig. 2(a) shows that permease activity is dependent on the sugar and
varies throughout fermentation. When the relative changes in permease activity in glucose and
maltose media were compared with the relative changes in pool size under the same conditions
(Fig. 2b), these two factors are seen to mirror each other up to 15 h, although the relative changes
in pool size are not as great as those in permease activity. These results support the suggestion
that ammonium permease activity is a major determinant of both the rate of ammonium uptake
and pool size, certainly in the first part of the fermentation. Further weight is given to this view
by a comparison between the observed rate of ammonium uptake during fermentation and that
calculated from the cell number and the ammonium permease specific activity (Fig. 3). For the
first 15 h, both sets of data have roughly similar values and show an upward trend in the case of
both glucose and maltose. Given the errors and assumptions involved in a comparison of this
type the agreement over the first 15 h seems quite good. The sharp fall-away in the calculated
uptake rate and divergence from the observed rate after 15 h cannot be explained at the moment
but it is clear that factors other than the ammonium permease activity must become
progressively more important in determining both uptake rate and pool size as the fermentation
proceeds.
It also appears that the sugar present in the medium affects the ammonium permease activity
and so, indirectly, exerts an influence on the rate of ammonium uptake and pool size in the first
part of the fermentation, but the mechanism of this phenomenon is unknown. The metabolism
of glucose and maltose is so similar that it seems unlikely that a sugar metabolite is involved, but
it is possible to speculate, as the two sugars are transported into the cell by different systems, that
interaction with ammonium uptake could occur at this level. It could be that continual synthesis
of the relatively unstable proteins involved in maltose transport and hydrolysis (Lagunas et al.,
1982) consumes a significant quantity of the ATP generated within the cell and so reduces the
amount of energy available to drive the active transport of compounds such as ammonium.
Another potential mechanism which may be worthy of consideration is that for any given
amount of hexose taken up, twice as many glucose molecules as compared to maltose molecules
have to be transported and, if there were an element of cotransport between sugar and
ammonium, this could provide an explanation for the effect of sugar on ammonium uptake.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Fri, 16 Jun 2017 21:22:16
NH,+ permease control of NHJ uptake by yeast
379
REFERENCES
BROWN,C. M. (1980). Ammonia assimilation and
utilization in bacteria and fungi. In Microorganisms
and Nitrogen Sources, pp. 511-535. Edited by J . W.
Payne. Chichester : Wiley.
DEVINE,
S. J. & SLAUGHTER,
J. C. (1980). The effect of
medium composition on the production of ethanol
by Saccharomyces cerevisiae. FEMS Microbiology
Letters 9, 19-21.
DUBOIS,
E., GRENSON,
M. & WIAME,J . M. (1974). The
participation of the anabolic glutamate dehydrogenase in the nitrogen catabolite repression of arginase
in Saccharomyces cerevisiae. European Journal of
Biochemistry 48, 603-616.
HARDING,
P. J . , OBI,E. I. &SLAUGHTER,
J. C. (1984).
The potential action of NH,+ as a stimulator of
ethanol production by Saccharomyces cerevisiae.
FEMS Microbiology Letters 21, 185-1 87.
LAGUNAS,
R., DOMINGUEZ,
C,, BUSTURIA,
A. & SAEZ,
M. J . (1982). Mechanisms of appearance of the
Pasteur effect in Saccharomyces cerevisiae : inactivation of sugar transport systems. Journal of Bacteriology 152, 19-25.
LLOYD,D., KRISTENSEN,
B. & DEGN, H. (1983).
Glycolysis and respiration in yeasts: the effect of
ammonium ions studied by mass spectroscopy.
Journal of General Microbiology 129, 2125-21 27.
ROON,R. J., EVEN,H. L., DUNLOP,P. & LARIMORE,
F.
L. (1975). Methylamine and ammonia transport in
Saccharomyces cerevisiae. Journal of Bacteriology 122,
502-509.
ROON,R. J., LEVY,J. S. & LARIMORE,
F. L. (1977a).
Negative interactions between amino acid and
methylamine/ammonia transport systems of Saccharomyces cerevisiae. Journal of Biological Chemistry
252, 3599-3604.
ROON,R. J., MEYER,G. M. & LARIMORE,
F. L. (19776).
Evidence for a common component in kinetically
distinct transport systems of Saccharomyces cerevisiae. Molecular and General Genetics 158, 185-1 9 1.
SAITA,M. (1983). Absorption of NH: by yeast. PhD
thesis, Heriot-Watt University.
J. C. (1984). Acceleration of
SAITA,M. & SLAUGHTER,
the rate of fermentation by Saccharomyces cereoisiae
in the presence of NHZ. Enzyme and Microbial
Technology 6 , 375-378.
SLAUGHTER,
J. C. & HOUSDEN,
S. J . (1978). Effect of
NH,+ on the production of ethanol and higher
alcohols by Saccharomyces cerevisiae. VIth International Specialized Symposium on Yeasts, Montpellier,
SII 7-8.
SOMOGYI,
M. & NELSON,N . (1944). A photometric
adaptation of the Somogyi method for determination
of sugars. Journal of Biological Chemistry 153, 375386.
THORNE,R. S. W. (1945). The mode of nitrogen
assimilation from mixed nutrients by growing yeast.
Journal of the Institute of Brewing 51, 6-17.
YOSHINO,
M. & MURAKAMI,
K. (1982). AMP deaminase reaction as a control system of glycolysis in yeast.
Activation of phosphofructokinase and pyruvate
kinase by the AMP deaminase-ammonia system.
Journal of Biological Chemistry 257,2822-2828.
Downloaded from www.microbiologyresearch.org by
IP: 88.99.165.207
On: Fri, 16 Jun 2017 21:22:16