In an effort to decipher the molecular mechanisms of homologous recombination during herpes simplex virus type-1 replication, we recently demonstrated that the virus-encoded single-stranded (ss) DNA-binding protein (ICP8) promotes the salt-dependent assimilation of ssDNA into a homologous plasmid, resulting in the formation of a displacement loop. In this paper, the results presented show for the first time a direct interaction between ICP8 and RNA. ICP8 binds to RNA with positive cooperativity but with ∼5-fold lower affinity than to ssDNA. In addition, competition experiments indicate that the dissociation rate of ICP8 from RNA is faster than from ssDNA, although it is also dependent on the nature of the challenger. Importantly, ICP8 can promote the salt-dependent assimilation of RNA into a homologous acceptor plasmid to generate a joint molecule in which the RNA is stably paired with the complementary strand of the acceptor DNA, indicative of an R-loop. These findings have important implications on the role of ICP8 in mediating recombination reactions using viral transcripts. The RNA-binding activity of ICP8 also provides a molecular basis for its role in the regulation of viral gene expression.
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