Transcription pattern of in vivo-labeled late simian virus 40 RNA: evidence that 16S and 19S mRNA's are derived from distinct precursor RNA populations.

The biosynthesis of the two major simian virus 40 mRNA molecules (19S mRNA and 16S mRNA) made at late times in the infective cycle was reinvestigated. By using a modified S1 nuclease technique, we were able to differentiate between pulse-labeled RNA precursor and the spliced mRNA. During a 5-min pulse-labeling with [3H]uridine in vivo, only precursor RNA molecules were detected. Experimental results with polyadenylic acid-selected 5-min pulse-labeled RNA are consistent with the notion that simian virus 40 late RNA can be polyadenylated before final splicing. Finally, 19S mRNA was spliced much more rapidly and appeared more quickly in the cytoplasm than 16S mRNA. Nevertheless, approximately one-half the precursor molecules were destined to become 16S mRNA. Thus, for at least these two viral mRNA's derived from a common transcription unit, the rate of splicing and the rate of nuclear exist are not major determinants of relative mRNA abundance.