Transcription of a poxvirus early gene is regulated both by a short promoter element and by a transcriptional termination signal controlling transcriptional interference.
The promoter region of an early gene (38K gene) of cowpox virus has been characterized by deletion and linker scanning mutational analyses. Modified versions of this promoter region were placed into the genome of vaccinia virus, and their transcriptional efficiencies were assessed by quantifying RNAs transcribed from these sequences. These analyses showed that the sequences in the region between 33 and 4 base pairs upstream of the transcriptional start site affect the efficiency of transcription from this promoter. Linker scanning mutations in the -27 to -10 region inhibited transcription. This region contains the sequence 5'-GAAAATATATT-3', which is present in at least two other early genes in the same positions (-21 to -11) relative to the transcriptional start sites of these genes. Elements of this sequence are similarly positioned in the promoter regions of several other poxvirus genes, suggesting that this sequence represents a transcriptional control element of at least a subset of poxvirus genes. The -8 to -2 sequence (5'-TTTTTAT-3') contains a transcriptional termination signal. Mutation of this sequence had two separate effects: (i) it reduced the efficiency of transcription from the promoter by approximately 30%, and (ii) it prevented this sequence from terminating the transcription from upstream genes. When overlapping transcription from upstream genes was not prevented by a termination signal present either within the 38K promoter or upstream of the promoter, transcription from this promoter was reduced by about 30%. This indicates that transcriptional termination has a role in the regulation of viral gene expression by controlling transcriptional interference.
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