Characterization of the genetic organization of the HindIII M region of the multicapsid nuclear polyhedrosis virus of Orgyia pseudotsugata reveals major differences among baculoviruses.

A region including the 4 kb HindIII M fragment of the multicapsid nuclear polyhedrosis virus (MNPV) of Orgyia pseudotsugata (OpMNPV) genome was sequenced, transcriptionally mapped, and compared to the homologous region in the MNPV of Autographa californica (AcMNPV). Five open reading frames (ORFs) oriented in the same direction were identified and were found to be expressed at late times post-infection. The mRNAs from the five ORFs were found to coterminate at a single site downstream of ORF 5. The conserved late gene promoter/mRNA start site sequence (AGTAAG) was present upstream of all the ORFs, but did not appear to be the major site of mRNA initiation for two of the ORFs as determined by primer extension analysis. These data indicated that use of this signal for transcriptional initiation may vary between different ORFs. The predicted amino acid sequences for the five ORFs of AcMNPV and OpMNPV were compared and amino acid homologies of 26 to 72% were observed. The comparison revealed a number of major differences in the genomes of the two viruses. A putative transposable element of 634 nucleotides was found to be inserted into the previously reported AcMNPV ORF 1 sequence. In addition, it was found that a region corresponding to the 4 kb HindIII K/EcoRI S/hr5 region of AcMNPV was not present in the OpMNPV genome.