Muniyappa K, Radding C M
J Biol Chem. 1986 Jun 5;261(16):7472-8.
The red genes of phage lambda specify two proteins, exonuclease and beta protein, which are essential for its general genetic recombination in recA- cells. These proteins seem to occur in vivo as an equimolar complex. In addition, beta protein forms a complex with another polypeptide, probably of phage origin, of Mr 70,000. The 70-kDa protein appears to be neither a precursor nor an aggregated form of either exonuclease or beta protein, since antibodies directed against the latter two proteins failed to react with 70-kDa protein on Ouchterlony double diffusion analysis. beta protein promotes Mg2+-dependent renaturation of complementary strands (Kmiec, E., and Holloman, W. K. (1981) J. Biol. Chem. 256, 12636-12639). To look for other pairing activities of beta protein, we developed methods of purification to free it of associated exonuclease. Exonuclease-free beta protein appeared unable to cause the pairing of a single strand with duplex DNA; however, like Escherichia coli single strand binding protein (SSB), beta protein stimulated formation of joint molecules by recA protein from linear duplex DNA and homologous circular single strands. Like recA protein, but unlike SSB, beta protein promoted the joining of the complementary single-stranded ends of phage lambda DNA. beta protein specifically protected single-stranded DNA from digestion by pancreatic DNase. The half-time for renaturation catalyzed by beta protein was independent of DNA concentration, unlike renaturation promoted by SSB and spontaneous renaturation, which are second order reactions. Thus, beta protein resembles recA protein in its ability to bring single-stranded DNA molecules together and resembles SSB in its ability to reduce secondary structure in single-stranded DNA.
噬菌体λ的红色基因编码两种蛋白质，即核酸外切酶和β蛋白，它们对于其在recA-细胞中的一般基因重组至关重要。这些蛋白质似乎在体内以等摩尔复合物的形式存在。此外，β蛋白与另一种可能源自噬菌体的70,000道尔顿的多肽形成复合物。70-kDa蛋白似乎既不是核酸外切酶也不是β蛋白的前体或聚集形式，因为在免疫双扩散分析中，针对后两种蛋白质的抗体未能与70-kDa蛋白发生反应。β蛋白促进互补链的Mg2+依赖性复性（Kmiec, E., and Holloman, W. K. (1981) J. Biol. Chem. 256, 12636 - 12639）。为了寻找β蛋白的其他配对活性，我们开发了纯化方法以去除其相关的核酸外切酶。不含核酸外切酶的β蛋白似乎无法使单链与双链DNA配对；然而，与大肠杆菌单链结合蛋白（SSB）一样，β蛋白刺激recA蛋白从线性双链DNA和同源环状单链形成连接分子。与recA蛋白一样，但与SSB不同，β蛋白促进噬菌体λ DNA互补单链末端的连接。β蛋白特异性保护单链DNA不被胰DNA酶消化。β蛋白催化复性的半衰期与DNA浓度无关，这与SSB促进的复性和自发复性不同，后者是二级反应。因此，β蛋白在使单链DNA分子聚集在一起的能力上类似于recA蛋白，在减少单链DNA二级结构的能力上类似于SSB。
