ALPHAHERPESVIRUS ORIGIN-BINDING PROTEIN HOMOLOG ENCODED BY HUMAN HERPESVIRUS 6B, A BETAHERPESVIRUS, BINDS TO NUCLEOTIDE-SEQUENCES THAT ARE SIMILAR TO ORI REGIONS OF ALPHAHERPESVIRUSES

We previously identified a human herpesvirus 6B (HHV-6B) homolog of th e alphaherpesvirus origin-binding protein (OBP), exemplified by the he rpes simplex virus type 1 UL9 gene product. This finding is of particu lar interest because HHV-6B is otherwise more closely related to membe rs of the betaherpesvirus subfamily. The prototypic betaherpesvirus, h uman cytomegalovirus, does not encode an obvious OBP homolog and conta ins a more complex origin of replication than do alphaherpesviruses. T hus, analysis of the function of the HHV-6B OBP homolog is essential f or understanding the mechanism of HHV-6B DNA replication initiation. T he HHV-6B OBP homolog, OBPH6B, was expressed in vitro by coupled trans cription and translation and in insect cells by infection with recombi nant baculoviruses. The expressed protein bound to two DNA sequences l ocated upstream of the HHV-6B major DNA-binding protein gene homolog, within a region that was predicted to serve as an origin of replicatio n on the basis of its sequence properties. The binding sites lie withi n 23-bp segments and are similar to OBP-binding sites of herpes simple x virus type 1. The two OBPH6B-binding sequences are separated by an A T-rich region and have an imperfect dyad symmetry as do the alphaherpe svirus origin regions. We identified OBPH6B transcripts by reverse tra nscription PCR in HHV-6B-infected Molt-3 cells. These results suggest that OBPH6B functions in a manner analogous to the alphaherpesvirus OB P and that initiation of HHV-6B DNA replication may resemble that of a lphaherpesviruses.