Two enhancers and one silencer located in the introns of regA control somatic cell differentiation in Volvox carteri

The regA gene plays a central role in germ-soma differentiation of Volvox c arteri by suppressing all reproductive functions in somatic cells. Here we show that the minimal promoter of regA consists of only 42 bp immediately u pstream of the transcription start site, and that it contains no discernibl e regulatory elements. However, introns 3 and 5 are both required for regA expression in somatic cells, and intron 7 is essential for silencing regA i n gonidia (asexual reproductive cells). A regA gene lacking intron 7 rescue s the normal phenotype of mutant somatic cells, but also results in gonidia that reproduce only weakly and soon die out. The same phenotype is observe d when a regA gene containing intron 7 is placed under control of a constit utive promoter, suggesting that the silencing activity of intron 7 is promo ter specific. Intron 7 is unusual in that it contains a potential ORE that is in frame with exons 7 and 8, and some transcripts are produced in which intron 7 is retained. However, a regulatory role for the intron 7 translati on product can be ruled out, because a construct in which intron 7 must be translated, and one in which it cannot be translated, both result in wild-t ype development of both cell types. Furthermore, intron 7 is unable to act in trans to silence regA, but is able to exert its normal effect when place d in a different location within the gene. Therefore, it appears that intro n 7 functions in gonidia as a classical cell-type-specific and promoter-spe cific enhancer, of the inhibitory type that is often referred to as a silen cer.