Unspliced functional late 19S mRNAs containing intervening sequences are produced by a late leader mutant of simian virus 40.

dl-805 is a viable deletion mutant of simian virus 40 lacking 187 of the 202 nucleotides coding for the principal leader of the wild-type viral late mRNAs. The structures of the mutant late 16S and 19S RNAs and proteins were examined with the following principal findings. (i) The vast majority, if not all, of the 19S RNAs are unspliced over their entire length. These RNAs contain intervening sequences and serve as mRNAs. The vast majority, if not all, of the 16S RNAs, however, retain the characteristic wild-type splice. (ii) Mutant 16S and 19S RNAs do not terminate at residue 243 (located six nucleotides upstream from the 5' end of the deleted DNA segment), the principal 5' terminus of the wild-type late mRNAs; rather, they begin at a series of sites, mostly upstream, used infrequently or not at all by wild-type virus. (iii) Despite the presence of altered late mRNAs, dl-805 late proteins have electrophoretic mobilities similar to those of wild-type late proteins, and mutant virions contain the same relative quantities of late proteins as wild-type virions.