Macromolecular Synthesis in Cells Infected by Frog Virus 3. VII. Transcriptional and Post-Transcriptional Regulation of Virus Gene Expression

Abstract

We have used improved techniques for separating individual species of RNA and protein to study the mechanisms that control gene expression by frog virus 3, a eucaryotic DNA virus. Forty-seven species of viral RNA and 35 viral polypeptide species were resolved by polyacrylamide gel electrophoresis. The relative molar ratios of virus-specific polypeptides synthesized at various times after infection were determined by computer planimetry and were compared with the molar ratios of appropriate-sized viral RNAs to code for each polypeptide. Viral polypeptides were classified according to the time during the growth cycle at which their maximal rate of synthesis occurred - early, 2 to 2.5 h; intermediate, 4 to 4.5 h; and late, 6 to 6.5 h. The viral RNAs, which were assumed to be mRNA's, could not be classified according to time of maximum synthesis; once their synthesis had begun, most of the RNAs continued to be synthesized at the same or higher rates. However, only 10 of the 47 viral RNA bands were plainly visible after electrophoresis of extracts from cells labeled from 1 to 1.5 h after infection; these 10 RNAs were designated “early” RNA. The early pattern of both RNA and polypeptide synthesis was maintained for at least 6 h in the presence of the amino acid analog fluorophenylalanine, which indicates that a functional viral polypeptide was required for “late” transcription and translation. The presumptive mRNA's for late polypeptides did not appear until 2 h after infection, but two of these “late” RNAs became the major products of transcription by 4 h into the infectious cycle. In contrast to the declining rate of synthesis of the early proteins, corresponding early RNA species continued to be synthesized at the same or higher rates throughout the replicative cycle. Although the synthesis of late virus-specific proteins appeared to be regulated at the level of transcription, our results suggest that the synthesis of both early and intermediate proteins was regulated at the post-transcriptional level.