Transcriptional analysis and preliminary characterization of ORF Bm42 from Bombyx mori nucleopolyhedrovirus

Clusters of closely spaced genes are frequently present in baculovirus genomes. We report here the detailed transcriptional mapping of one such gene cluster from Bombyx mori nucleopolyhedrovirus (BmNPV) comprising five ORFs, viz., Bm42, Lef-10, VP1054, Bm44, and Bm45, which are closely spaced and transcribed in the same orientation. This region is homologous to ORFs 53, 53a, 54, 55, and 56, respectively, on the AcMNPV genome. Multiple, overlapping sets of polycistronic transcripts from this region, that initiate independently but terminate at a common 3' end, were detected following BmNPV infection. Expression of these genes followed a temporal pattern where the shorter transcripts initiating downstream appeared early, and with the progress of infection the longer transcripts initiated from upstream. The first ORF in this cluster, Bm42, is conserved in almost all baculoviruses, but has not been characterized so far. The protein was localized in the cytosol, predominantly near the nucleus-cytoplasm boundary in BmNPV-infected BmN cells. Bm42 was present in the budded virions and was not associated with occluded virus particles or the polyhedral matrix.