The Moloney Murine Sarcoma Virus ts110 59 Splice Site Signal Contributes to the Regulation of Splicing Efficiency

The 5 * splice site signal (5*ss) in Moloney murine sarcoma virus ts110 (MuSVts110) RNA was found to participate in the regulation of its splicing phenotype. This 5*ss (CAG/GUAGGA) departs from the mammalian consensus (CAG/GURAGU) at positions 14 and 16, both of which base pair with U1 and U6 small nuclear RNAs during splicing. A doubling in splicing efficiency and near elimination of the splicing thermosensitivity characteristic of MuSVts110 were observed in 5*ss mutants containing a U at position16 (termed 5*A6U), even in those in which U1-5*ss complementarity had been reduced. At the permissive temperature (28&C), the 5*A6U mutation increased the efficiency of the second splicing reaction, while at the nonpermissive temperature (39&C), both splicing reactions were positively affected. Retroviral replication requires incomplete splicing of the genome length primary transcript. Unspliced RNA is trans-lated into gag and pol gene products and used as genomic RNA for the packaging of virions, while spliced RNA is translated into env gene products. For several retroviral pre-mRNAs, it has been shown that suboptimal splicing signals near the 39 splice site signal (39ss) mediate the balance between unsplice