THE MOLONEY MURINE SARCOMA-VIRUS TS110 5' SPLICE-SITE SIGNAL CONTRIBUTES TO THE REGULATION OF SPLICING EFFICIENCY AND THERMOSENSITIVITY

The 5' splice site signal (5'ss) in Moloney murine sarcoma virus ts110 (MuSVts110) RNA was found to participate in the regulation of its spl icing phenotype. This 5'ss (CAG/GUAGGA) departs from the mammalian con sensus (CAG/GURAGU) at positions +4 and +6, both of which base pair wi th U1 and U6 small nuclear RNAs during splicing. A doubling in splicin g efficiency and near elimination of the splicing thermosensitivity ch aracteristic of MuSVts110 were observed in 5'ss mutants containing a U at position +6 (termed 5'A6U), even in those in which U1-5'ss complem entarity had been reduced. At the permissive temperature (28 degrees C ), the 5'A6U mutation increased the efficiency of the second splicing reaction, while at the nonpermissive temperature (39 degrees C), both splicing reactions were positively affected.