TEMPERATE MYXOCOCCUS-XANTHUS PHAGE MX8 ENCODES DNA ADENINE METHYLASE,MOX

Temperate bacteriophage Mx8 of Myxococcus xanthus encapsidates termina lly repetitious DNA, packaged as circular permutations of its 49-kbp g enome. During both lytic and lysogenic development, Mx8 expresses a no nessential DNA methylase, Mox, which modifies adenine residues in occu rrences of XhoI and PstI recognition sites, CTCGAG and CTGCAG, respect ively, on both phage DNA and the host chromosome. The mox gene is nece ssary for methylase activity in vivo, because an amber mutation in the mox gene abolishes activity. The mox gene is the only phage gene requ ired for methylase activity in vivo, because ectopic expression of mox as part of the M. xanthus mg/BA operon results in partial methylation of the host chromosome. The predicted amino acid sequence of Mox is r elated most closely to that of the methylase involved in the cell cycl e control of Caulobacter crescentus. We speculate that Mox acts to pro tect Mx8 phage DNA against restriction upon infection of a subset of n atural M. xanthus hosts. One natural isolate of M. xanthus, the lysoge nic source of related phage Mx81, produces a restriction endonuclease with the cleavage specificity of endonuclease BstBI.