G1 regulation and checkpoints operating around START in fission yeast.

Three major aspects of G1 regulation acting at START in fission yeast are discussed in this review. Firstly, progression towards S phase in the mitotic cycle. This is controlled by the activation of transcription 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 G1. Passage through START appears to inhibit sexual differentiation because the meiotic and mitotic pathways are mutually exclusive. This is brought about because the meiotic pathway is inhibited by the same gene functions that are required for S phase onset. Thirdly, distinct checkpoint, or dependency, controls 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.