The regulatory networks of gene expression during the sexual differentiation of Chlamydomonas reinhardtii, as analyzed by mutants for gametogenesis.

Cells of Chlamydomonas reinhardtii differentiate into gametes under conditions of nitrogen (N) starvation, expressing the genes for the N-adaptation program and the gamete program. To investigate the regulatory networks of transcription among the N-starvation-inducible genes, we examined the gene expression in dif mutants, affecting gametic differentiation. In a conditional mutant, dif2, the cells remained 'vegetative' at the restrictive temperature, and the induction of 20 out of 21 genes related to the two programs was impaired. They were expressed soon after transfer of the cells to the permissive temperature, in parallel with the acquisition of mating ability. In an unconditional mutant, dif3, the cells could not differentiate into gametes at all, but the induction of only four genes (FUS1, NSG3, NSG6 and NSG7) related to the gamete program was impaired. The results suggest that Dif3 regulates putative N-starvation signal transduction pathways downstream of a master regulator, Dif2. We also examined a light-dependent laboratory strain that was unable to become gametes in the dark. The 'pre-gametes' placed in the dark, however, could induce normally all of the 21 genes, suggesting that light is required for the gametic differentiation at the translational and/or post-translational levels.