Genetic Determinants of Immunity and Integration of Temperate Myxococcus xanthus Phage Mx8

An 8.1-kb fragment of the temperate Myxococcus xanthus phage Mx8 genome, when cloned into a plasmid vector, permits site-specific integration of the plasmid and confers superinfection immunity. Sequence analysis of a 9.5-kb region of Mx8 DNA containing this fragment reveals 19 densely packed open reading frames, four of which have predicted products with known or suspected activities. The Mx8 imm gene, required for superinfection immunity, has a sequence similar to that of Arabidopsis thaliana G-box-binding factor 1. Mx8 makes a DNA adenine methylase, Mox, and integrase, Int, related to other methylases and integrases. The int gene has two alternate translation initiation codons within the extensively overlapping uoi (upstream of int) gene. Comparison of the predicted product of the uoi gene with Salmonella phage P22 and Streptomyces plasmid Xis proteins shows that temperate phage excisionases may use variations of a helix-turn-helix motif to recognize specific DNA sequences.

将温和型黄色粘球菌（Myxococcus xanthus）噬菌体Mx8基因组的8.1 kb片段克隆至质粒载体（plasmid vector）后，该片段可介导质粒发生位点特异性整合，并赋予宿主超感染免疫性。对包含该片段的Mx8 DNA 9.5 kb区域进行序列分析，共鉴定出19个紧密排布的开放阅读框（open reading frame），其中4个的预测编码产物具有已知或疑似的生物学活性。负责介导超感染免疫的Mx8 imm基因，其序列与拟南芥（Arabidopsis thaliana）G盒结合因子1（G-box-binding factor 1）具有序列同源性。Mx8可编码DNA腺嘌呤甲基化酶（DNA adenine methylase）Mox与整合酶（integrase）Int，二者与其他同类甲基化酶和整合酶具有同源相关性。int基因在与uoi（int基因上游）基因高度重叠的区域内，存在两个交替使用的翻译起始密码子（translation initiation codons）。将uoi基因的预测编码产物与沙门氏菌（Salmonella）噬菌体P22及链霉菌（Streptomyces）质粒Xis蛋白进行序列比对后发现，温和噬菌体的切除酶可能通过螺旋-转角-螺旋（helix-turn-helix）基序的变异形式来识别特定的DNA序列。