PROTEIN-SYNTHESIS IN LONG-TERM STATIONARY-PHASE CULTURES OF SACCHAROMYCES-CEREVISIAE

We are interested in characterizing the process of entry into and the maintenance of the stationary phase. To identify proteins that are ind uced during growth to stationary phase, we examined protein synthesis in long-term stationary-phase cultures using two-dimensional polyacryl amide gel electrophoresis (2D-PAGE). Although the total rate of protei n synthesis declined when growth ceased after the postdiauxic phase, t he pattern of proteins synthesized remained similar throughout the exp erimental period (28 days), except at the diauxic shift. At the diauxi c shift most proteins detectable by 2D-PAGE undergo a transient reduct ion in their relative rate of synthesis that ends when cells resume gr owth during the postdiauxic phase. We conclude from this that the tran sient repression of protein synthesis at the diauxic shift is not dire ctly associated with stationary-phase arrest. A number of proteins tha t are synthesized after exponential phase have been identified by 2D-P AGE. These proteins could be divided into three temporal classes depen ding upon when their synthesis became detectable. One postexponential protein, designated p35, was induced later than all other proteins, an d its relative rate of synthesis increased throughout stationary phase . Unlike most postexponential proteins, p35 was not regulated by heat shock or glucose repression. We also observed that a direct correlatio n between steady-state mRNA accumulation and protein synthesis for ano ther postexponential protein (Ssa3p) or two closely related constituti ve proteins (Ssa1p and Ssa2p) did not exist. We conclude from this res ult that synthesis of proteins in stationary phase is regulated by mec hanisms other than the control of steady-state mRNA accumulation.