Kim S H, Chung Y B
Institute of Molecular Biology, Paik Hospital, Inje University 85, Seoul, South Korea.
Virology. 1996 Feb 1;216(1):20-5. doi: 10.1006/viro.1996.0030.
Half of the 55 potential genes of bacteriophage T7 appear to be dispensable. One of the major obstacles in the study of these nonessential genes is the difficulty in obtaining mutants. During a study of genes involved in the packaging of bacteriophage T7, we hypothesized that some nonessential genes may be required for optimal growth. Mutant phages lacking such nonessential genes may form plaques but grow slowly. One gene located at the extreme right end of the linear T7 genome, gene 19.5 with no known mutants, and a genetic element m responsible for a unique hairpin end, were studied. Mutant T7 phages deleted in gene 19.5 and m (T7 delta 19.5-M) were generated in vivo by homologous recombination with a recombinant plasmid. This phage produces small plaques and the production of progeny phage particles per infected cell was reduced fourfold. Investigation of the intracellular DNA after infection with T7 delta 19.5-M showed the persistence of Escherichia coli DNA as well as delayed conversion of concatemers to unit-length T7 DNA. The inefficiency of concatemer processing confirmed the proposed function of the M-hairpin in duplication of the concatemer junction. Since it is not likely that the M-hairpin influences the degradation of host DNA, we propose that the gene 19.5 product is partly responsible for the degradation of E. coli chromosomal DNA.
噬菌体T7的55个潜在基因中，似乎有一半是可有可无的。研究这些非必需基因的主要障碍之一是难以获得突变体。在一项关于噬菌体T7包装相关基因的研究中，我们推测某些非必需基因可能是最佳生长所必需的。缺乏此类非必需基因的突变噬菌体可能会形成噬菌斑，但生长缓慢。我们研究了位于线性T7基因组最右端的一个基因（基因19.5，尚无已知突变体）以及一个负责独特发夹末端的遗传元件m。通过与重组质粒进行同源重组，在体内产生了基因19.5和m缺失的突变T7噬菌体（T7 delta 19.5-M）。这种噬菌体产生小噬菌斑，每个感染细胞产生的子代噬菌体颗粒数量减少了四倍。用T7 delta 19.5-M感染后对细胞内DNA的研究表明，大肠杆菌DNA持续存在，并且串联体向单位长度T7 DNA的转化延迟。串联体加工效率低下证实了M发夹在串联体连接复制中的假定功能。由于M发夹不太可能影响宿主DNA的降解，我们提出基因19.5产物部分负责大肠杆菌染色体DNA的降解。
