Lin H, Black L W
Department of Biochemistry, University of Maryland Medical School, Baltimore 21201-1503, USA.
Virology. 1998 Mar 1;242(1):118-27. doi: 10.1006/viro.1997.9019.
Phage T4 terminase, comprising the products of genes 16 and 17, packages headfuls of DNA from a concatemer but its mechanism of DNA recognition remains to be determined. Phage T4 terminase gene sequences were introduced into prophage lambda imm434 and plasmids in order to assess their effect on packaging as measured by transduction frequency and DNA content of T4-transducing particles. Multiple copy prophage lambda imm434 genes were transduced at 100-fold higher frequency, and high copy plasmids were transduced at 1000-fold higher frequency than single copy prophage or chromosomal genes T4 16 gene inserts enhanced both prophage and plasmid packaging; terminase gene-containing plasmid DNA in T4 transducing particles could exceed 10% of the total. Deletion or base change of the 24-bp gene 16 3' region which is required for sequence specific amplification of terminase gene 17 (Hp 17 mutations) depressed these elevated plasmid transduction frequencies, suggesting that this is a preferred T4 pac sequence. Moreover, a specific gene 16-containing pac fragment could be detected in mature, packaged phage T4 DNA following restriction endonuclease digestion. We conclude that both the copy number of homologous sequences and the DNA pac sequence(s) themselves are important for packaging, consistent with a synapsis model for regulation of terminase cutting and packaging in phage T4.
噬菌体T4的末端酶由基因16和17的产物组成，它从串联体中包装满基因组的DNA，但其DNA识别机制仍有待确定。将噬菌体T4末端酶基因序列导入原噬菌体λ imm434和质粒中，以便通过转导频率和T4转导颗粒的DNA含量来评估它们对包装的影响。多拷贝的原噬菌体λ imm434基因的转导频率比单拷贝原噬菌体或染色体基因高100倍，高拷贝质粒的转导频率比单拷贝原噬菌体或染色体基因高1000倍。T4 16基因插入片段增强了原噬菌体和质粒的包装；T4转导颗粒中含末端酶基因的质粒DNA可超过总量的10%。对末端酶基因17序列特异性扩增所必需的24 bp基因16 3'区域进行缺失或碱基改变（Hp 17突变），会降低这些升高的质粒转导频率，这表明这是T4的一个优选pac序列。此外，在成熟的、包装好的噬菌体T4 DNA经限制性内切酶消化后，可检测到一个特定的含基因16的pac片段。我们得出结论，同源序列的拷贝数和DNA pac序列本身对包装都很重要，这与噬菌体T4中末端酶切割和包装调控的联会模型一致。
