A highly conserved genomic region in baculoviruses: sequence analysis of an 11.3 kbp DNA fragment (46.5-55.1 m.u.) of the Spodoptera exigua multicapsid nucleopolyhedrovirus.

A DNA fragment of 11.3 kilobase pairs (kbp) in size of the baculovirus Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) genome (46.5 to 55.1 m.u.) was completely sequenced. Analysis of the sequence revealed eleven potential open reading frames (ORF). Ten of these ORFs showed significant amino acid identity to Autographa californica MNPV (AcMNPV) and Orgyia pseudotsugata MNPV (OpMNPV) genes p6.9, lef5, 38K, p19, p143, p25, p18, vp33, lef4, and vp39. One ORF (XC12) has no homolog in other baculoviruses and may be unique to SeMNPV. All but three of these putative genes are preceded by the consensus baculovirus late promoter element (5'-ATAAG-3'). The genetic organization and the putative map of transcripts of this fragment suggested that this region is highly similar to a region in AcMNPV fragment EcoRI-D. Comparison of the genetic organization of this 11.3 kbp fragment with the genomes of AcMNPV, OpMNPV, Bombyx mori NPV (BmNPV) and SeMNPV revealed that this region is highly conserved among baculovirus genomes. This is in contrast to the genetic organization of the polyhedrin-p10 region, which is much more diverged, but has been taken as point of reference to orient baculovirus physical maps. Through its diversity the latter region, however, would be an excellent candidate to determine baculovirus relatedness and phylogeny. The presence of conserved and diverged regions in baculovirus genomes with respect to gene order is reminiscent to the situation in other large DNA viruses, such as herpes- and poxviruses, where conserved central and diverged terminal parts are common characteristics. The role of this feature in the genomic organization of large DNA viruses is discussed with particular emphasis on virus replication and evolution.