Volvox has two cell types: mortal somatic cells and immortal germ cells. Here we describe the transposon-tagging, cloning and characterization of regA, which plays a central role as a master regulatory gene in Volvox germ-soma differentiation by suppressing reproductive activities in somatic cells. The 12.5 kb regA transcription unit generates a 6,725 nucleotide mRNA that appears at the beginning of somatic cell differentiation, and that encodes a 111 kDa RegA protein that localizes to the nucleus, and has an unusual abundance of alanine, glutamine and proline. This is a compositional feature shared by functional domains of many ‘active’ repressors. These findings are consistent with the hypothesis that RegA acts in somatic cells to repress transcription of genes required for growth and reproduction, including 13 genes whose products are required for chloroplast biogenesis.

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