regA, a Volvox gene that plays a central role in germ-soma differentiation, encodes a novel regulatory protein

Volvox has two cell types: mortal somatic cells and immortal germ cells. He re we describe the transposon-tagging, cloning and characterization of regA , which plays a central role as a master regulatory gene in Volvox germsoma differentiation by suppressing reproductive activities in somatic cells. T he 12.5 kb regA transcription unit generates a 6,725 nucleotide mRNA that a ppears at the beginning of somatic cell differentiation, and that encodes a 111 kDa RegA protein that localizes to the nucleus, and has an unusual abu ndance of alanine, glutamine and proline, This is a compositional feature s hared 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 g enes whose products are required for chloroplast biogenesis.