Control of cell division by a retinoblastoma protein homolog in Chlamydomonas

A key pathway that controls both cell division and differentiation in anima l cells is mediated by the retinoblastoma (RB) family of tumor suppressors, which gate the passage of cells from G, to S and through S phase. The role (s) of the RE pathway in plants are not yet clearly defined, nor has there been any evidence for its presence in unicellular organisms. Here we have i dentified an RE homolog encoded by the mat3 gene in Chlamydomonas reinhardt ii, a unicellular green alga in the land plant lineage. Chlamydomonas cells normally grow to many times their original size during a prolonged G, and then undergo multiple alternating rounds of S phase and mitosis to produce daughter cells of uniform size. mat3 mutants produce small daughter cells a nd show defects in two size-dependent cell cycle controls: They initiate th e cell cycle at a below-normal size, and they undergo extra rounds of S pha se/mitosis. Unlike mammalian RE mutants, mat3 mutants do not have a shorten ed G,, do not enter S phase prematurely, and can exit the cell cycle and di fferentiate normally, indicating that the RE pathway in Chlamydomonas has a different role than in animals.