Biotecnologia

JOURNAL OF BACTERIOLOGY, Jan. 1979, p. 1-5 Vol. 137, No. 1
0021-9193/79/01-0001/05$02.00/0
Regulation of Cell Size in the Yeast Saccharomyces cerevisiae
G. C. JOHNSTON,`* C. W. EHRHARDT,1 A. LORINCZ,2 AND B. L. A. CARTER2
Department ofMicrobiology, Dalhousie University, Halifax, Nova Scotia, Canada,' and Department of
Genetics, Trinity College, University ofDublin, Dublin 2, Ireland2Received for publication 21 September 1978
For celLs of the yeast Saccharomyces cerevisiae, the size at initiation of budding
is proportional to growth rate for rates from 0.33 to 0.23 h-1. At growth rates
lower than 0.23 h-1, cells displayed a minimum cell size at bud initiation independent
of growth rate. Regardless of growth rate, cells displayed an increase in
volume each time budding wasinitiated. When abnormally small cells, produced
by starvation for nitrogen, were placed in fresh medium containing nitrogen but
with different carbon sources, they did not initiate budding until they had grown
to the critical size characteristic of that medium. Moreover, when cells were
shifted from a medium supporting a low growth rate and small size at bud
initiation to a medium supporting ahigher growth rate and larger size at bud
initiation, there was a transient accumulation of cells within Gl. These results
suggest that yeast cells are able to initiate cell division at different cell sizes and
that regulation of cell size occurs within Gl.
Yeast cells in culture maintain a constant cell
size through coordination between the processes
of growth and cell division (3, 10).Growth does
not normally occur in the absence of cell division,
producing very large cells, nor does division
normally continue in the absence of growth,
producing very small cells. Recently, we suggested
the following scheme for the coordination
of growth and cell division in the yeast Saccharomyces
cerevisiae: the completion of an essential
event in the Gl phase of the cell cycle
requiresgrowth to some critical size (8, 10), and
growth is always rate limiting for progression
through the cell division cycle (10). We noted
that under conditions of nutrient starvation
small Gl-arrested cells were produced. When
placed in fresh medium, these cells grew to a
critical size before initiating a cell division cycle.
These observations suggested that, regardless of
growth conditions,a cell must attain some critical
size before initiation of cell division may
occur. The yeast is a particularly suitable organism
for such study, since the initiation of cell
division or DNA synthesis is coincident with the
onset of budding (15) (thus, cells within Gl are
unbudded).
We have now determined if the same critical
size is required for initiation of cell division by
cellsgrowing at different rates. We report that
under different growth conditions yeast cells are
able to initiate cell division at different cell sizes.
MATERIALS ANDI METHODS
Strains and growth conditions. The yeast S.
cerevisiae was studied. Haploid cells of strain C4.2
(derived from the diploid C276, kindly provided by J.
R. Pringle) were cultured at a variety of steady-state
growth rates in aglucose-limited medium in a chemostat
(L. H. Engineering Co., Ltd., Bucks, England)
at 240C (7). The diploid strain AG1-7 and the isogenic
haploid GR2 have been described elsewhere (11).
Cells were also grown in a minimal medium (yeast
nitrogen base without ammonium sulfate or amino
acids [YNB]; Difco Laboratories, Detroit, Mich.) and
an enriched medium (YM1). Both media have beenpreviously described (10).
Photomicroscopy and cell volume determination.
Cells were collected either from chemostat samples
or from batch culture by centrifugation and suspended
in 1 ml of a solution of Calcofluor (American
Cyanamide Co., Pearl River, N. Y.) (6). The concentration
of Calcofluor used was either 2 (Table 1) or 10
(Tables 2 and 3) mg/ml in water. Stained cells were
photographed...