The B1 gene of vaccinia virus encodes a serine/threonine protein kinase that is expressed early after infection. Under nonpermissive conditions, temperature-sensitive mutants (ts2 and ts25) that map to B1 fail to efficiently replicate viral DNA. Studies on the regulation of intracellular processes involved in eukaryotic nucleic acid metabolism have pointed to protein phosphorylation as playing a key role. Poxviruses have acquired analogs of a number of host cell genes to maintain their autonomy from the cell, including two serine/threonine protein kinases (B1 and F10) and a dual-specificity protein phosphatase (H1). The F10 kinase (22) and H1 phosphatase (15) are expressed late in infection, are incorporated into virions, and play major roles in virion morphogenesis (11,24,38). Interestingly, the H1 phosphatase has been implicated in early gene transcription (24). Mutant virus particles devoid of H1 phosphatase were unable to transcribe early genes either in vivo or in vitro. This finding led us to investigate the role that phosphorylation might play in the transcription of the other gene classes. The B1 kinase is expressed exclusively early in infection; it localizes to viral replication factories and appears to be a minor component of the virion (3,23,34

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