Saccharomyces cerevisiae colony growth and ageing: Biphasic growth accompanied by changes in gene expression

Although colony growth and morphology are central tools in yeast genetics, little is known about the cell physiology and how it changes during the col ony growth and ageing. Here we show that the growth of a well-separated Sac charomyces cerevisiae colony is biphasic; a rapid growth phase is followed by a sharp transition to a slower growth phase. In the first growth phase ( similar to 24 cell divisions) most, if not all, cells divide at a rate simi lar to that in liquid medium and exhibit morphological, biochemical and gen etic characteristics of cells engaged in the cell cycle. During the second growth phase, cells in the centre of a colony gradually enter stationary ph ase, so that later in this phase the growth occurs predominantly at the per iphery. Unlike the biphasic growth in rich liquid media containing a fermen table carbon source, in which the first growth is fuelled by fermentation a nd the second by aerobic metabolism, the two phases of the colony growth ca n be fuelled either exclusively by fermentation or exclusively by aerobic m etabolism. We also describe a novel technique for in situ estimation of the transcriptional status in the colony cells, which was used to monitor tran scription dynamics during the colony development. Using this technique and standard methods to determine mRNA levels, we show that the transition betw een the first and second growth phases is accompanied by a global change in the pattern of transcription: transcription of most genes is repressed whi le that of some genes is induced. Copyright (C) 1999 John Wiley & Sons, Ltd .