Translational regulation of SV40 early mRNA defines a new viral protein.

SP6-initiated in vitro transcripts, representing the three major classes of early SV40 mRNAs, early-early (EE) and two late-early (LE) transcripts, were assayed by in vitro translation to compare their relative efficiencies for synthesis of the SV40 T antigens. The presence of one or two potential AUG initiator codons in the leader sequences of the LE RNAs inhibits efficient translation from the downstream T-antigen initiator AUG. In vitro translation of the capped form of the shorter SV40 LE RNA resulted in the synthesis of a 2.7-kd protein. In vivo pulse labeling of SV40-infected CV-1 cells demonstrated the accumulation of a peptide of similar size at late times after lytic infection, indicating that it is an authentic viral protein encoded by the early leader sequence of SV40.