Nuclear processing of viral high-molecular-weight RNA in cells infected with herpes simplex virus type 1

Relation between the levels of mRNA abundance and kinetics of protein synthesis in pseudorabies virus-infected cells

Virus proteins synthesized by pseudorabies virus-infected cells can be classified into five groups on the basis of the kinetics of their synthesis at various stages of the infective process; virus mRNAs can similarly be classified into four groups. To determine whether the kinetics of synthesis of specific proteins are determined solely by the level of abundance in the cells of their mRNAs, we have compared at various times after infection the relative synthesis of these proteins with the relative abundance of their mRNAs. We have focused on two proteins: the 142K major capsid protein, an early-late protein, and the 136K major DNA binding protein, an early protein. The mRNAs encoding these proteins were identified. The relative abundances of these mRNAs in the cytoplasms of the infected cells were found to be the same as those associated with the polysome fractions. The relative amount of the proteins synthesized by the infected cells at a given stage of the infective process closely reflected the relative amount of the mRNA encoding these proteins that was present in the cells at that stage of the infective process. Most virus mRNA species that are present in the cytoplasm of infected cells were represented on polysomes to approximately an equal extent. Some RNA species were, however, significantly underrepresented under certain conditions in the polysomal fractions. We conclude that whereas the amount of many virus proteins synthesized by the infected cells is determined mainly by the level of the abundance of their mRNAs, additional controls operate in the cells that determine the relative rates of synthesis of some other virus proteins.

The large late virus transcripts synthesized in herpesvirus suis (pseudorabies) virus-infected cells are not precursors of mRNA

The sizes of early and late pseudorabies virus transcripts were compared to those of early and late mRNA. While the early primary transcripts were of approximately the same size as early mRNA, a large proportion of late primary transcripts was much larger than late mRNA. Furthermore, most early transcripts were transported efficiently to the cytoplasm and were relatively stable. In contrast, a large proportion of the late transcripts were retained in the nucleus and turned over rapidly. Specific retention in the nucleus of transcripts originating from some regions of the genome could be detected. These were, however, not preferentially degraded; degradation of transcripts originating from all regions of the genome, including those from which late mRNA originates, occurred. Experiments designed to determine whether part of the large transcripts are processed into mRNA revealed that most of the large late transcripts synthesized by the infected cells bear no precursor relationship to mRNA. Thus, during late phase of infection, most regions of the genome are abundantly transcribed as large RNA molecules; these are not destined to be processed into mRNA.

Post-transcriptional control of human cytomegalovirus gene expression

During the immediate-early, early, and late phases of human cytomegalovirus infection in human fibroblasts, transcripts accumulated, respectively, from approximately 20, 75, and 90% of the sequences on the genome. Not all of the sequences which accumulated during the immediate-early and early phases were represented on polysomes, however. Four transcripts synthesized in cycloheximide-treated cells were studied in detail. A 2.2-kb transcript (0.713-0.733 map units) represented 95% of the polysome-associated RNA in cycloheximide-treated cells and was the first to be detected on polysomes at 2 hr postinfection in untreated cells. A second, less abundant, transcript of 5.2 kb (0.670-0.733 map units) was also found on polysomes in cycloheximide-treated cells, and preliminary evidence suggested that this transcript may be spliced during processing. A 3.25-kb transcript (0.190-0.217 map units) was identified also as a minor polysome-associated species of RNA. One transcript of 4.8 kb (0.630-0.670) remained associated with the nucleus and was not processed into mRNA in cycloheximide-treated cells. Differential stability between the various transcripts was observed, the 2.2-kb transcript being the most stable. The results showed that in human cytomegalovirus-infected cells controls exist at the level of transcript accumulation, transport into the cytoplasm, preferential association with polysomes, and relative stability of RNAs.