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Southern blot analysis of Volvox carteri DNA indicated the presence of a single actin gene; the nucleotide sequence of that gene is reported here. In comparison with plant animal and fungal actins, the derived primary structure of 377 amino acids is highly conserved yielding similarity values of 79% to 94% (including non-identical conservative exchanges). In contrast, the intron structure of the gene is highly unusual: in addition to one intron in the 5' untranslated region (ten nucleotides upstream of the initiator ATG), it has eight introns in the coding region, only three of which are in locations where introns have previously been reported. Transcription starts 26 nucleotides downstream of the putative TATA box and 70 nucleotides downstream of a conspicuous CCAAT motif. A potential polyadenylation signal, TGTAA, is located 366 nucleotides downstream of the terminator TAA. Northern hybridization indicates that the actin gene is transcribed throughout the Volvox life cycle with only a slight depression during the release of juveniles from mother spheroids. This pattern of gene expression suggests that actin may assume various functional roles in the differentiation and growth of Volvox.

In Volvox carteri, regA acts as a master gene to suppress all germ cell functions in somatic cells. Its product, RegA, has features of a transcriptional repressor. Here we report cDNA sequences representing 15 nuclear genes with properties expected of RegA targets: they are expressed strongly in germ cells and in regA-, but not regA+, somatic cells. Two of them encode polypeptides with no recognizable features, but ten (like three previously sequenced ones) encode chloroplast proteins of known function, and the remaining three encode putative chloroplast proteins of unknown function. This suggests that RegA blocks reproductive development in somatic cells by preventing chloroplast biogenesis, thereby making it impossible for the cells to grow enough to reproduce.