Oberto J, Weisberg R A, Gottesman M E
Laboratory of Molecular Genetics, National Institute of Child Health and Human Development, Bethesda, MD 20892.
J Mol Biol. 1989 Jun 20;207(4):675-93. doi: 10.1016/0022-2836(89)90237-4.
The immunity region of the lambdoid phage, HK022, has been sequenced. The HK022 repressor gene, its cognate operators and promoters, and several early phage genes can be discerned. The overall design of the immunity region resembles that of other lambdoid phages. The location of the HK022 nun gene, whose product excludes superinfecting lambda by terminating transcription at (or near) the lambda nut sites, is analogous to that of gene N in lambda. nun is preceded by sequences similar to the lambda nut sites and the lambda pL promoter and is followed by several transcription termination signals. Despite these similarities, Nun is required neither for the lytic nor the lysogenic pathway of phage development. Again, unlike N, Nun is expressed in a prophage, perhaps from a promoter other than pL. We suggest that Nun and N have diverged in evolution and now perform different functions for their respective phages. Although Nun and N compete at the lambda nut sites and interact with the same host Nus proteins, they are only distantly related in predicted amino acid sequence. The presence of transcription terminators in the pL operon suggests that the expression of the HK022 early functions, like those of lambda, entails an antitermination mechanism. However, Nun does not appear to be an essential component of this mechanism. Our most economic model is that the HK022 nutL sequence suppresses pL operon terminators in the absence of a phage-encoded antitermination protein. Striking homologies between the HK022 nutL sequence and related sequences in the Escherichia coli rrn operons support this notion. Alternatively, a phage antitermination gene may be located outside the pL operon.
λ样噬菌体HK022的免疫区域已完成测序。可以识别出HK022阻遏蛋白基因、其同源操纵基因和启动子，以及几个早期噬菌体基因。免疫区域的总体设计与其他λ样噬菌体相似。HK022 nun基因的位置类似于λ噬菌体中的基因N，其产物通过在λ噬菌体nut位点（或其附近）终止转录来排除超感染的λ噬菌体。nun基因之前是与λ噬菌体nut位点和λ噬菌体pL启动子相似的序列，之后是几个转录终止信号。尽管有这些相似之处，但噬菌体发育的裂解途径和溶原途径都不需要Nun。同样，与N不同，Nun在原噬菌体中表达，可能来自pL以外的启动子。我们认为，Nun和N在进化过程中发生了分化，现在为各自的噬菌体执行不同的功能。虽然Nun和N在λ噬菌体nut位点竞争并与相同的宿主Nus蛋白相互作用，但它们在预测的氨基酸序列上只有远缘关系。pL操纵子中存在转录终止子表明，HK022早期功能的表达，与λ噬菌体一样，需要一种抗终止机制。然而，Nun似乎不是这种机制的必要组成部分。我们最经济的模型是，在没有噬菌体编码的抗终止蛋白的情况下，HK022 nutL序列抑制pL操纵子终止子。HK022 nutL序列与大肠杆菌rrn操纵子中的相关序列之间显著的同源性支持了这一观点。或者，噬菌体抗终止基因可能位于pL操纵子之外。
