In the cell cycle of the budding yeast Saccharomyces cerevisiae, expression of the histone genes H2A and H2B of the TRT1 and TRT2 loci is regulated by the performance of "start," the step that also regulates the cell cycle. Here we show that histone production is also subject to an additional form of regulation that is unrelated to the mitotic cell cycle. Expression of histone genes, as assessed by Northern (RNA) analysis, was shown to increase promptly after the stimulation, brought about by fresh medium, that activates stationary-phase cells to reenter the mitotic cell cycle. The use of a yeast mutant that is conditionally blocked in the resumption of proliferation at a step that is not part of the mitotic cell cycle (M.A. Drebot, G.C. Johnston, and R.A. Singer, Proc. Natl. Acad. Sci. 84:7948, 1987) showed that this increased gene expression that occurs upon stimulation of stationary-phase cells took place in the absence of DNA synthesis and without the performance of start. This stimulation-specific gene expression was blocked by the mating pheromone alpha-factor, indicating that alpha-factor directly inhibits expression of these histone genes, independently of start.
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