Synthesis of stable unspliced mRNA from an intronless simian virus 40--rat preproinsulin gene recombinant.

We constructed a recombinant DNA molecule carrying the coding region and the 3' flanking sequences of the rat preproinsulin I gene, replacing the late region of a simian virus 40 (SV40) viral vector. The sequences making up the single intron of the preproinsulin I gene, located within the nontranslated leader region, as well as all of the 5' flanking sequences, have been deleted. The sequences remaining within the late transcriptional unit of SV40 do not retain a functional intron. This "intronless" mutant nevertheless directs the synthesis of substantial quantities of unspliced stable hybrid transcripts that are translated into polypeptides with insulin immunoreactivity. Two novel 5' termini, not used in wild-type SV40 transcripts, have been detected in the majority of the hybrid RNA molecules. These are located at analogous positions in both of the late SV40 72-base-pair repeats.