Regulation of delayed-early gene transcription by dual TATA boxes

Abstract

The 39K Autographa californica nuclear polyhedrosis virus (AcMNPV) gene is highly expressed throughout the virus life cycle and is controlled by tandem promoters that exhibit features of early and late baculovirus promoters. Late transcripts initiate at a conserved TAAG motif, while early transcripts are heterogeneous and initiate near a conserved CAGT motif. To define the nucleotide sequences that regulate early transcription of the 39K gene, a series of mutations was generated by substitution of 10-bp stretches in the 39K promoter with a BglII linker. The effects of these mutations on transcription from the early promoter were determined by transient expression and primer extension assays in the presence of the viral trans-activator IE1 gene. Mutations in the region from -15 to -44 revealed that early 39K transcription was controlled by dual TATA boxes. These TATA boxes are separated by 10 bp, which partially accounts for the heterogeneity in early 39K transcripts. Transcripts initiating at the CAGT motif (proximal transcripts) were abolished by deletion of the proximal TATA box located at -29 relative to CAGT. Proximal transcripts were not affected by alterations in the distal TATA motif located at -39 relative to the CAGT. Similarly, transcripts initiating upstream of CAGT (distal transcripts) were eliminated by mutations in the distal TATA but were unaffected by substitutions in the proximal TATA box. Proximal transcripts were not detected with a plasmid containing mutations in the CAGT motif, although the distal transcripts were unaffected by CAGT mutations. When the sequences surrounding the initiation site for the distal transcripts were altered, the start site was shifted one nucleotide, but transcription was not quantitatively affected. These results suggest that early 39K transcription is controlled by two distinct TATA elements, one that is dependent on an initiator and one in which the site of initiation is determined by the TATA element alone. Mutations in an upstream region from -45 to -68 relative to the CAGT motif had a quantitative effect but did not alter the heterogeneous pattern of early transcripts, suggesting these sequences function as an upstream regulatory region. Analysis of late transcription indicated that the TAAG element was essential, while transcription was unaffected by other mutations.