DENSONUCLEOSIS VIRUSES CONSTITUTE AN INCREASINGLY DIVERSIFIED SUBFAMILY AMONG THE PARVOVIRUSES

Densoviruses continue to increase in, number and, as a group, to diversify. Based on recent data on the structure and organization of their genome, we propose here a taxonomic structure for the Densovirinae subfamily including four genera instead of the three adopted recently by the ICTV. One putative genus, Bidensovirus, is proposed for densoviruses with a bipartite genome which replicate differently from other parvoviruses. A working model of their replication fitting the characteristics of the genome is proposed. Densoviruses share most of the physicochemical characteristics of the vertebrate counterparts. In addition, several of them have striking sequence motifs in common with vertebrate parvovirus structural and nonstructural genes. Despite these similarities, densoviruses use expression strategies which are quite different from those of vertebrate parvoviruses, thus justifying an independent subfamily. First, important transcription factor and promoter sequence motif differences are noted with respect to vertebrate parvoviruses and, secondly, recognition of particular sequences around the initiator codon determines the fraction of scanning ribosomes using it. In contrast, (alternative) splicing determines the extent Of expression of overlapping genes of vertebrate parvoviruses. Finally, we discuss the use of densoviruses as tools in biological control and as expression vectors.