Franklin J L, Mosig G
Department of Molecular Biology, Vanderbilt University, Nashville, TN 37235, USA.
Gene. 1996 Oct 24;177(1-2):179-89. doi: 10.1016/0378-1119(96)00299-5.
The products of the bacteriophage T4 terminase genes 16 and 17 are known to mediate cutting and packaging of concatemeric vegetative DNA. We show here that the larger of these genes, 17, yields multiple protein species. The complex expression of the T4 terminase genes includes overlapping transcripts, probably initiated from multiple promoters, RNA processing at certain preferred sites and translation initiation from multiple ribosome binding sites (RBS). Translation initiation from these RBS may be modulated by inverted repeat (IR) sequences whose folding can be predicted to differ in different RNA species. In T4 infected bacteria, genes 16 and 17 are probably co-transcribed from several near-consensus late promoters upstream from gene 16, and processed at multiple sites. Additional 5' ends of late transcripts are located downstream from a near-consensus late promoter inside gene 17 and further downstream, unrelated to any known promoter consensus sequence. The gene 17 transcripts that are initiated or cleaved internally contain RBS for shorter open reading frames (ORFs) in the same frame as full-length gene product (gp) 17 of 70 kDa. The truncated proteins, a 59-kDa gp17' and a 45-kDa gp17", are synthesized from cloned gene 17 segments in which the first gene 17 RBS is deleted. Expression of gene 17 is different in BL21(DE3) or W3110[pACT7] host bacteria. The gp17' and gp17" proteins are predicted to contain one or more of the ATPase motifs that are common among large subunits of other phage terminases. They lack a predicted single stranded (ss) DNA binding motif that is unique the large terminase proteins in T4 gp17, and that has been implicated in recognizing ssDNA regions in replicating and recombining T4DNA destined to be packaged. We hypothesize that a truncated gene 17' is an evolutionary precursor of the full-size T4 gene 17. Its function may have been maintained to allow processive packaging from double stranded (ds) DNA ends.
已知噬菌体T4终止酶基因16和17的产物介导多联体营养DNA的切割和包装。我们在此表明，这两个基因中较大的基因17产生多种蛋白质。T4终止酶基因的复杂表达包括重叠转录本，可能由多个启动子起始，在某些特定位点进行RNA加工，并从多个核糖体结合位点（RBS）起始翻译。从这些RBS起始的翻译可能受反向重复（IR）序列调控，这些序列的折叠在不同RNA种类中预计有所不同。在T4感染的细菌中，基因16和17可能从基因16上游几个接近共有序列的晚期启动子共同转录，并在多个位点加工。晚期转录本的额外5'末端位于基因17内部一个接近共有序列的晚期启动子下游以及更下游位置，与任何已知的启动子共有序列无关。内部起始或切割的基因17转录本含有与70 kDa全长基因产物（gp）17同一阅读框内较短开放阅读框（ORF）的RBS。截短蛋白，一个59 kDa的gp17'和一个45 kDa的gp17''，由克隆的基因17片段合成，其中第一个基因17的RBS被删除。基因17在BL21(DE3)或W3110[pACT7]宿主细菌中的表达不同。预计gp17'和gp17''蛋白含有其他噬菌体终止酶大亚基中常见的一个或多个ATP酶基序。它们缺乏预测的单链（ss）DNA结合基序，该基序是T4 gp17中大型终止酶蛋白所特有的，并且与识别注定要包装的复制和重组T4DNA中的ssDNA区域有关。我们假设截短的基因17'是全长T4基因17的进化前体。其功能可能得以保留，以允许从双链（ds）DNA末端进行持续包装。
