Zhang W, Evans D H
Department of Molecular Biology and Genetics, University of Guelph, Ontario, Canada.
Nucleic Acids Res. 1995 Nov 25;23(22):4620-7. doi: 10.1093/nar/23.22.4620.
A protein which promotes DNA strand transfer between linear double-stranded M13mp19 DNA and single-stranded viral M13mp19 DNA has been isolated from recA- E.coli. The protein is DNA polymerase I. Strand transfer activity residues in the small fragment encoding the 5'-3' exonuclease and can be detected using a recombinant protein comprising the first 324 amino acids encoded by polA. Either the recombinant 5'-3' exonuclease or intact DNA polymerase I can catalyze joint molecule formation, in reactions requiring only Mg2+ and homologous DNA substrates. Both kinds of reactions are unaffected by added ATP. Electron microscopy shows that the joint molecules formed in these reactions bear displaced single strands and therefore this reaction is not simply promoted by annealing of exonuclease-gapped molecules. The pairing reaction is also polar and displaces the 5'-end of the non-complementary strand, extending the heteroduplex joint in a 5'-3' direction relative to the displaced strand. Thus strand transfer occurs with the same polarity as nick translation. These results show that E.coli, like many eukaryotes, possesses a protein which can promote ATP-independent strand-transfer reactions and raises questions concerning the possible biological role of this function.
一种能促进线性双链M13mp19 DNA与单链病毒M13mp19 DNA之间DNA链转移的蛋白质已从recA⁻大肠杆菌中分离出来。该蛋白质是DNA聚合酶I。链转移活性存在于编码5'-3'核酸外切酶的小片段中，并且可以使用包含polA编码的前324个氨基酸的重组蛋白进行检测。在仅需要Mg²⁺和同源DNA底物的反应中，重组5'-3'核酸外切酶或完整的DNA聚合酶I均可催化接头分子形成。这两种反应均不受添加的ATP影响。电子显微镜显示，在这些反应中形成的接头分子带有置换的单链，因此该反应并非简单地由核酸外切酶缺口分子的退火促进。配对反应也是有极性的，会置换非互补链的5'-末端，相对于置换链，以5'-3'方向延伸异源双链接头。因此，链转移与切口平移具有相同的极性。这些结果表明，大肠杆菌与许多真核生物一样，拥有一种能促进不依赖ATP的链转移反应的蛋白质，并引发了关于该功能可能的生物学作用的问题。
