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.

Molecular characterization and pathogenicity of a grouper iridovirus (GIV) isolated from yellow grouper, Epinephelus awoara (Temminck & Schlegel)

“…6). It has been suggested that this protein comprises 40% of the virion (Kelly & Tinsley 1972; Willis et al. 1977; Black et al.…”

“…One of the unifying features of this virus group is the presence of a major capsid protein (MCP) of approximately 50 kDa. This is the predominant polypeptide produced upon viral infection and can account for up to 45% of virion protein (Kelly & Tinsley 1972; Willis, Goorha, Miles & Granoff 1977; Black, Blair, Butcher, Capinera & Happ 1981). The MCP genes have been fully sequenced from the invertebrate iridoviruses, insect iridescent virus type 22 (IV22), Tipula iridescent virus (TIV), Chilo iridescent virus (CIV), Wiseana iridescent virus (WIV) and Costelytra zealandica iridescent virus (CzIV) (Cameron 1990; Tajbakhsh, Lee, Watson & Seligy 1990; Stohwasser, Raab, Schnitzler, Janssen & Darai 1993; Webby & Kalmakoff 1999), and the vertebrate iridoviruses, lymphocystis disease virus (LCDV)‐1 and FV3 (Schnitzler & Darai 1993; Mao, Tham, Gentry, Aubertin & Chinchar 1996).…”

Macromolecular Synthesis in Cells Infected by Frog Virus 3 XII. Viral Regulatory Proteins in Transcriptional and Post-Transcriptional Controls

“…or late (i.e., synthesized at 2 h p.i. and thereafter), because once the synthesis of any viral RNA species had begun, its rate was maintained at the same or at a higher level (13).…”

“…This drug, when incorporated into proteins in an essential site, renders such proteins nonfunctional without inhibiting protein synthesis in general (11). The addition of FPA at the beginning of infection had pleiotropic effects (13). Only the early RNAs were synthesized, even at 6 to 8 h p.i.…”

“…Since both early and intermediate proteins were synthesized in the presence of FPA, we have now elected to classify all of these proteins as early, to correspond with the early RNAs. However, as these two groups of proteins displayed obvious differences in both the time of their own peak rate of synthesis (13) and the time of synthesis of their regulatory proteins (see Results), we have subdivided them into class I proteins or class II proteins. The difference between early and late proteins is a qualitative one; only early proteins are present at 1 h p.i.…”