Examination of Signals for Initiation of DNA Synthesis on Single Strand, and their Consensus Nucleotide Sequences

质粒pACYC 184、RSF 1010和pSY 343的基因组携带一个或两个ssi信号。它们的大小和结构各不相同。由相同酶系统操作的信号在结构上常常不同。然而，与ssi信号迄今已确定，两个结构特征被发现是共同的。首先，ssi信号决定序列含有与在大肠杆菌中发现的共有序列5^1-GAAGCGG-3^1同源的核苷酸序列。coli N^1蛋白识别位点。第二，潜在的茎环结构包含在ssi区域。尽管这些共同的结构特征似乎起着重要的作用，但它们对ssi功能的贡献模式还没有被揭示出来。目前研究中发现的ssi信号在功能模式和生物学意义上是多样的，可以分为三类。第一类; ssi信号位于oriV区域，并指导引物RNA的合成，为DNA链的延伸提供先导 关于我们 ing链（RSF 1010的ssiA和ssiB，以及pSY 343的ssiA）。第二类; ssi信号与引导前导链起始的区域分开定位，并且似乎引导滞后链DNA合成（pACYC 184的ssi）。第三类; ssi信号位于远离引导链起始的区域，可能在质粒接合传递过程中引导DNA合成，或在营养期DNA复制过程中有效地进行链延伸（pSY 343的ssiB）中起一定作用。由于ssi信号的多样性反映了复制子的进化过程，ssi信号将成为寻找DNA复制机制起源的有力标准，DNA复制机制的功能最终归结于ssi信号，ssi信号直接参与提供DNA链延伸所需的RNA引物。在未来，许多其他ssi信号应被检查，以了解整个结构和功能方面的ssi信号，并确定特定的生物学特性的结构要素应被揭示。这方面的研究应该得到进一步的发展，以便有可能设计出适合特定目的的人工DNA复制机器。少

Genomes of plasmids pACYC184, RSF1010 and pSY343 carries one or two ssi signals. They are diverse in their size and structure. Signals which are operated by the same enzyme system are often different in their structure. However, with the ssi signals thus far identified, two structural features are found in common. First, the ssi signal-determining sequences contain nucleotide sequences homologous to a concensus sequence 5^1-GAAGCGG-3^1 found in the E. coli N^1 protein recognition sites. Second, potential stem-loop structures are contained in the ssi regions. Although the common structural features seem to play important roles, their mode of contribution to ssi function is not revealed yet.The ssi signals identified in the present studies are diverse in their mode of function and biological significance, and can be grouped into three classes. The first class; the ssi signals which are located in the oriV region, and direct the synthesis of primer RNA for DNA chain elongation of the lead … More ing strand (ssiA and ssiB of RSF1010, and ssiA of pSY343). The second class; the ssi signals which are located separately from the region directing the leading strand initiation, and seem to direct the lagging strand DNA synthesis (ssi of pACYC184). The third class; the ssi signals which are located far apart from the region directing the leading strand initiation, and might direct the DNA synthesis during conjugative transmission of plasmids, or might play some roles for efficient progress of chain elongation furing vegetative DNA replication (ssiB of pSY343). Because the diverse biological properties of the ssi signals reflect the evolutional process of replicons, ssi signals will be a powerful criterion of searching for origins of DNA-replicating machinery.The function of DNA-replicating machinery is ultimately attributed to that of ssi signals which is directly involved in providing RNA primers for DNA chain elongation. For the future, many other ssi signals should be examined to understand the whole structural and functional aspect of ssi signals, and structural elements determining the specific biological properties should be revealed. Studies on this line should be advanced for making it possible to design artificial DNA-replicating machinery suiting with specific purposes. Less