Sandegren Linus, Nord David, Sjöberg Britt-Marie
Department of Molecular Biology and Functional Genomics, Stockholm University, Svante Arrhenius väg 16, F3. SE-10691, Stockholm, Sweden.
Nucleic Acids Res. 2005 Oct 27;33(19):6203-13. doi: 10.1093/nar/gki932. Print 2005.
T4 contains two groups of genes with similarity to homing endonucleases, the seg-genes (similarity to endonucleases encoded by group I introns) containing GIY-YIG motifs and the mob-genes (similarity to mobile endonucleases) containing H-N-H motifs. The four seg-genes characterized to date encode homing endonucleases with cleavage sites close to their respective gene loci while none of the mob-genes have been shown to cleave DNA. Of 18 phages screened, only T4 was found to have mobC while mobE genes were found in five additional phages. Interestingly, three phages encoded a seg-like gene (hereby called segH) with a GIY-YIG motif in place of mobC. An additional phage has an unrelated gene called hef (homing endonuclease-like function) in place of the mobE gene. The gene products of both novel genes displayed homing endonuclease activity with cleavage site specificity close to their respective genes. In contrast to intron encoded homing endonucleases, both SegH and Hef can cleave their own DNA as well as DNA from phages without the genes. Both segH and mobE (and most likely hef) can home between phages in mixed infections. We discuss why it might be a selective advantage for phage freestanding homing endonucleases to cleave both HEG-containing and HEG-less genomes.
T4 含有两组与归巢内切酶相似的基因，即seg基因（与 I 组内含子编码的内切酶相似），含有GIY - YIG 基序，以及mob基因（与可移动内切酶相似），含有H - N - H 基序。迄今为止鉴定出的四个seg基因编码的归巢内切酶，其切割位点靠近各自的基因座，而尚未有mob基因被证明能切割DNA。在筛选的18种噬菌体中，仅发现T4 具有mobC，而在另外五种噬菌体中发现了mobE基因。有趣的是，三种噬菌体编码了一个具有GIY - YIG 基序的类seg基因（在此称为segH）来替代mobC。另外一种噬菌体具有一个名为hef（类归巢内切酶功能）的不相关基因来替代mobE基因。这两个新基因的产物均显示出归巢内切酶活性，其切割位点特异性靠近各自的基因。与内含子编码的归巢内切酶不同，SegH 和Hef 既能切割自身的DNA，也能切割不含这些基因的噬菌体的DNA。segH 和mobE（很可能还有hef）在混合感染的噬菌体之间都能归巢。我们讨论了为什么噬菌体独立的归巢内切酶切割含HEG和不含HEG的基因组可能具有选择优势。
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