The simplest extensively studied photosynthesizing organism that contains the conserved elements of the RB-E2F pathway is the unicellular green alga, Chlamydomonas reinhardtii. Recent genome sequencing analysis of this alga revealed that it has single copy genes encoding homologues of RB/MAT3, E2F and DP; therefore it provides the simplest model for the analysis of the RB-E2F pathway (Bisova et al. 2005; and another chapter in this volume). RB (mat3) mutant algae cells undergo supernumerous divisions, while mutations in dp1 or in e2f1 suppress this ability (Fang et al. 2006; Umen and Goode-nough 2001). These data suggest a similar function for RB/MAT3 as its animal counterparts in repressing E2F-DP function. Since rb/mat3/e2f1/dp1mutant algae could divide more or less normally, the RB-E2F pathway might not be essential for cell division in Chlamydomonas. However, cell size control was abrogated in the mat3-e2f-dp mutant; rb/mat3 divide at an extremely small size, while dp1 and certain e2f1 mutants became larger than wild-type, but the molecular mechanism behind these observations is not well understood.
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