Generation of authentic 3' termini of an H2A mRNA in vivo is dependent on a short inverted DNA repeat and on spacer sequences.

We have determined what sequences are required to generate the authentic 3' termini of a sea urchin H2A histone mRNA. We have constructed a series of deletion and insertion mutants in the cloned histone repeat unit h22 of Psammechinus miliaris and have analyzed the transcripts of both wild-type and mutant DNAs produced in the frog oocyte. The protein-coding sequences of the H2A gene can be removed without any deleterious effects on transcription initiation or termination. A 12 bp deletion, which removes a highly conserved inverted DNA repeat immediately preceding the H2A mRNA 3' terminus, elicits read-through of the polymerase into the spacer DNA further downstream. However, the inverted repeat and the sequence coding for the 3' terminus of the mRNA are by themselves not sufficient to generate faithful 3' ends. Our data suggest that spacer sequences downstream of the 3' mRNA terminus are required as well.