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

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Abstract
Improved techniques were used 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 RNA 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, 2-2.5 h; intermediate, 4-4.5 h; and late, 6-6.5 h. The viral RNA, which were assumed to be mRNA could not be classified according to time of maximum synthesis; once their synthesis had begun, most of the RNA continued to be synthesized at the same or higher rates. Only 10 of the 47 viral RNA bands were plainly visible after electrophoresis of extracts from [fat head minnow, FHM] cells labeled from 1-1.5 h after infection; these 10 RNA 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 for late polypeptides did not appear until 2 h after infection, but 2 of these late RNA became the major products of transcription by 4 h into the infectious cycle. Unlike 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, the synthesis of both early and intermediate proteins was probably regulated at the post-transcriptional level.