G(1) REGULATION AND CHECKPOINTS OPERATING AROUND START IN FISSION YEAST

Three major aspects of G(1) regulation acting at START in fission yeas t are discussed in this review, Firstly, progression towards S phase i n the mitotic cycle. This is controlled by the activation of transcrip tion complexes at START which cause cell cycle-dependent activation of genes required for DNA synthesis. The second aspect is the regulation of developmental fate occurring during G(1). Passage through START ap pears to inhibit sexual differentiation because the meiotic and mitoti c pathways are mutually exclusive. This is brought about because the m eiotic pathway is inhibited by the same gene functions that are requir ed for S phase onset. Thirdly distinct checkpoint, or dependency, cont rols operate both pre- and post-START in the mitotic cycle to inhibit mitosis in the absence of replicated DNA, and also to limit rounds of DNA replication to one per cell cycle.