Yang X, McFadden B A
Department of Biochemistry and Biophysics, Washington State University, Pullman 99193-4660.
Plasmid. 1994 Mar;31(2):131-7. doi: 10.1006/plas.1994.1014.
Both strands of small plasmid pCB2.4 from the unicellular cyanobacterium Synechocystis PCC 6803 have been sequenced and analyzed. pCB2.4 contains 2345 bp and 57% A+T. Three open reading frames (ORFs) were identified in the nucleotide sequence of one strand of pCB2.4. ORF1 and ORF3 may encode basic proteins with calculated pI values of 7.71 and 9.96, respectively. The products of these ORFs have been compared to published protein sequences in databases and do not show significant homology to other proteins including replication proteins. pCB2.4 is not similar at the global DNA level to another 2.4-kbp plasmid, pCA2.4, present in the same cells, which has been found to replicate by a rolling-circle mechanism via a single-stranded intermediate. However, a short region (37 bp) is similar in pCA2.4 and pCB2.4. This region contains a potential nicking site (GATA) for replication proteins encoded by a group of plasmids that replicate by a rolling-circle mechanism and might be used as the origin of replication for these plasmids. Detection of a low level of single-stranded intermediate for pCB2.4 in the Synechocystis 6803 suggests that pCB2.4 may also replicate by a rolling-circle mechanism.
对单细胞蓝藻集胞藻PCC 6803中的小质粒pCB2.4的两条链进行了测序和分析。pCB2.4含有2345个碱基对，A+T含量为57%。在pCB2.4一条链的核苷酸序列中鉴定出三个开放阅读框（ORF）。ORF1和ORF3可能分别编码计算出的pI值为7.71和9.96的碱性蛋白。这些ORF的产物已与数据库中已发表的蛋白质序列进行比较，与包括复制蛋白在内的其他蛋白质没有显著同源性。pCB2.4在整体DNA水平上与同一细胞中存在的另一个2.4-kbp质粒pCA2.4不相似，pCA2.4已被发现通过单链中间体以滚环机制复制。然而，pCA2.4和pCB2.4中有一个短区域（37个碱基对）是相似的。该区域含有一个潜在的切口位点（GATA），用于一组通过滚环机制复制的质粒所编码的复制蛋白，可能被用作这些质粒的复制起点。在集胞藻6803中检测到pCB2.4的低水平单链中间体，这表明pCB2.4也可能通过滚环机制复制。
