Sirisena D M, MacLachlan P R, Liu S L, Hessel A, Sanderson K E
Department of Biological Sciences, University of Calgary, Alberta, Canada.
J Bacteriol. 1994 Apr;176(8):2379-85. doi: 10.1128/jb.176.8.2379-2385.1994.
We report the analysis of three open reading frames of Salmonella typhimurium LT2 which we identified as rfaF, the structural gene for ADP-heptose:LPS heptosyltransferase II; rfaD, the structural gene for ADP-L-glycero-D-manno-heptose-6-epimerase; and part of kbl, the structural gene for 2-amino-3-ketobutyrate CoA ligase. A plasmid carrying rfaF complements an rfaF mutant of S. typhimurium; rfaD and kbl are homologous to and in the same location as the equivalent genes in Escherichia coli K-12. The RfaF (heptosyl transferase II) protein shares regions of amino acid homology with RfaC (heptosyltransferase I), RfaQ (postulated to be heptosyltransferase III), and KdtA (ketodeoxyoctonate transferase), suggesting that these regions function in heptose binding. E. coli contains a block of DNA of about 1,200 bp between kbl and rfaD which is missing from S. typhimurium. This DNA includes yibB, which is an open reading frame of unknown function, and two promoters upstream of rfaD (P3, a heat-shock promoter, and P2). Both S. typhimurium and E. coli rfaD genes share a normal consensus promoter (P1). We postulate that the yibB segment is an insertion into the line leading to E. coli from the common ancestor of the two genera, though it could be a deletion from the line leading to S. typhimurium. The G+C content of the rfaLKZYJI genes of both S. typhimurium LT2 and E. coli K-12 is about 35%, much lower than the average of enteric bacteria; if this low G+C content is due to lateral transfer from a source of low G+C content, it must have occurred prior to evolutionary divergence of the two genera.
我们报告了对鼠伤寒沙门氏菌LT2三个开放阅读框的分析，我们将其鉴定为rfaF，即ADP-庚糖：LPS庚糖基转移酶II的结构基因；rfaD，即ADP-L-甘油-D-甘露庚糖-6-表异构酶的结构基因；以及kbl的一部分，即2-氨基-3-酮丁酸辅酶A连接酶的结构基因。携带rfaF的质粒可互补鼠伤寒沙门氏菌的rfaF突变体；rfaD和kbl与大肠杆菌K-12中的等效基因同源且位于相同位置。RfaF（庚糖基转移酶II）蛋白与RfaC（庚糖基转移酶I）、RfaQ（推测为庚糖基转移酶III）和KdtA（酮脱氧辛酸转移酶）共享氨基酸同源区域，表明这些区域在庚糖结合中起作用。大肠杆菌在kbl和rfaD之间包含一段约1200 bp的DNA片段，而鼠伤寒沙门氏菌中缺失该片段。这段DNA包括yibB，它是一个功能未知的开放阅读框，以及rfaD上游的两个启动子（P3，一个热休克启动子，和P2）。鼠伤寒沙门氏菌和大肠杆菌rfaD基因都共享一个正常的共有启动子（P1）。我们推测yibB片段是在从这两个属的共同祖先通向大肠杆菌的谱系中插入的，尽管它也可能是在通向鼠伤寒沙门氏菌的谱系中缺失的。鼠伤寒沙门氏菌LT-2和大肠杆菌K-12的rfaLKZYJI基因的G+C含量约为35%，远低于肠道细菌的平均值；如果这种低G+C含量是由于从低G+C含量来源的横向转移所致，那么它一定发生在这两个属的进化分歧之前。
