Characterization of Spodoptera litura multicapsid nucleopolyhedrovirus 38.7k protein, which contains a conserved BRO domain

Homology analysis revealed that Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) 38.7k protein has 22–83% amino acid identities with Ecotropis obliqua NPV, Mamestra configurata MNPV, Helicoverpa armigera SNPV, H. zea SNPV, S. exigua MNPV and S. littoralis MNPV 38.7k proteins. Analysis of the relationship of these 38.7k proteins indicated that they contain a conserved BRO-N domain, and SpltMNPV and SpliMNPV 38.7k proteins also contain a motif found in all known viral and prokaryotic single-strand DNA binding proteins. RT-PCR results showed that SpltMNPV 38.7k gene is transcribed actively at the late stage of infection and the mRNA start site was mapped within a consensus baculovirus late promoter motif (ATAAG). Western blot analysis revealed that the 38.7k was expressed in infected S. litura cells as a 41 kDa form and this protein distributed in the nucleus of infected cells. Using a histone extraction protocol, SpltMNPV 38.7k could be detected in the histone H1 fraction. Micrococcal nuclease treatment released SpltMNPV 38.7k protein from the chromatin fraction, suggesting that its involvement in nucleosome structures. Furthermore, column chromatography using DNA-cellulose showed that SpltMNPV 38.7k protein interacted with nucleic acids. It was proposed that SpltMNPV 38.7k might function as a DNA-binding protein.