Lohman T M, Ferrari M E
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
Annu Rev Biochem. 1994;63:527-70. doi: 10.1146/annurev.bi.63.070194.002523.
There are now several well-documented SSBs from both prokaryotes and eukaryotes that function in replication, recombination, and repair; however, no "consensus" view of their interactions with ssDNA has emerged. Although these proteins all bind preferentially and with high affinity to ssDNA, their modes of binding to ssDNA in vitro, including whether they bind with cooperativity, often differ dramatically. This point is most clear upon comparing the properties of the phage T4 gene 32 protein and the E. coli SSB protein. Depending on the solution conditions, Eco SSB can bind ssDNA in several different modes, which display quite different properties, including cooperativity. The wide range of interactions with ssDNA observed for Eco SSB is due principally to its tetrameric structure and the fact that each SSB protomer (subunit) can bind ssDNA. This reflects a major difference between Eco SSB and the T4 gene 32 protein, which binds DNA as a monomer and displays "unlimited" positive cooperativity in its binding to ssDNA. The Eco SSB tetramer can bind ssDNA with at least two different types of nearest-neighbor positive cooperativity ("limited" and "unlimited"), as well as negative cooperativity among the subunits within an individual tetramer. In fact, this latter property, which is dependent upon salt concentration and nucleotide base composition, is a major factor influencing whether ssDNA interacts with all four or only two SSB subunits, which in turn determines the type of intertetramer positive cooperativity. Hence, it is clear that the interactions of Eco SSB with ssDNA are quite different from those of T4 gene 32 protein, and the idea that all SSBs bind to ssDNA as does the T4 gene 32 protein must be amended. Although it is not yet known which of the Eco SSB-binding modes is functionally important in vivo, it is possible that some of the modes are used preferentially in different DNA metabolic processes. In any event, the vastly different properties of the Eco SSB-binding modes must be considered in studies of DNA replication, recombination, and repair in vitro. Since eukaryotic mitochondrial SSBs as well as SSBs encoded by prokaryotic conjugative plasmids are highly similar to Eco SSB, these proteins are likely to show similar complexities. However, based on their heterotrimeric subunit composition, the eukaryotic nuclear SSBs (RP-A proteins) are significantly different from either Eco SSB or T4 gene 32 proteins. Further subclassification of these proteins must await more detailed biochemical and biophysical studies.
现在已经有来自原核生物和真核生物的几种记录详尽的单链结合蛋白（SSB），它们在复制、重组和修复过程中发挥作用；然而，关于它们与单链DNA（ssDNA）相互作用的“共识”观点尚未形成。尽管这些蛋白质都优先且高亲和力地结合ssDNA，但它们在体外与ssDNA的结合模式，包括是否协同结合，往往有很大差异。在比较噬菌体T4基因32蛋白和大肠杆菌SSB蛋白的特性时，这一点最为明显。根据溶液条件，大肠杆菌SSB（Eco SSB）可以以几种不同模式结合ssDNA，这些模式表现出截然不同的特性，包括协同性。Eco SSB与ssDNA观察到的广泛相互作用主要归因于其四级结构以及每个SSB原体（亚基）都能结合ssDNA这一事实。这反映了Eco SSB与T4基因32蛋白之间的一个主要差异，T4基因32蛋白以单体形式结合DNA，并且在与ssDNA结合时表现出“无限”的正协同性。Eco SSB四聚体可以以至少两种不同类型的近邻正协同性（“有限”和“无限”）结合ssDNA，以及在单个四聚体内亚基之间存在负协同性。事实上，后一种特性取决于盐浓度和核苷酸碱基组成，是影响ssDNA与所有四个还是仅两个SSB亚基相互作用的主要因素，这反过来又决定了四聚体间正协同性的类型。因此，很明显Eco SSB与ssDNA的相互作用与T4基因32蛋白的截然不同，那种认为所有SSB都像T4基因32蛋白那样结合ssDNA的观点必须修正。虽然尚不清楚Eco SSB的哪种结合模式在体内具有功能重要性，但有可能某些模式在不同的DNA代谢过程中被优先使用。无论如何，在体外DNA复制、重组和修复研究中必须考虑Eco SSB结合模式的巨大差异特性。由于真核生物线粒体SSB以及原核生物接合质粒编码的SSB与Eco SSB高度相似，这些蛋白质可能表现出类似的复杂性。然而，基于其异源三聚体亚基组成，真核生物核SSB（RP - A蛋白）与Eco SSB或T4基因32蛋白都有显著差异。对这些蛋白质的进一步分类必须等待更详细的生化和生物物理研究。
