In vivo and in vitro expression of U7 snRNA genes: cis- and trans-acting elements required for RNA polymerase II-directed transcription.

Three of five genes coding for U7 small nuclear (sn) RNA of the sea urchin Psammechinus miliaris were shown to be expressed during early embryogenesis by microinjection into sea urchin eggs followed by fertilization. Both in vivo and in nuclear extracts of blastula embryos, a minimal promoter of 80 bp of 5' flanking sequence is essential for their expression. Sequences upstream of position -80 enhance transcription in vivo, but not in vitro, approximately 5-fold. In vitro, transcription initiates at nucleotide +1 of the U7 snRNA and is directed by RNA polymerase II. Protein-DNA binding studies and site-directed mutagenesis demonstrate the presence of multiple proteins interacting with sequences between -57 and -26, which are essential for selection of the correct initiation site and for efficient in vitro transcription. Three of these factors recognize a TATA-like regulatory element between positions -53 and -45, suggesting a role for TATA-binding proteins in the initiation of sea urchin U7 snRNA transcription.