Indian Institute of Chemical Biology (CSIR), Kolkata 700 032, India.
National Institute of Cholera and Enteric Diseases (ICMR), Kolkata 700 010, India.
Microbiology (Reading). 2010 Jan;156(Pt 1):99-107. doi: 10.1099/mic.0.032458-0. Epub 2009 Oct 1.
The ctxAB operon, encoding cholera toxin (CT) in Vibrio cholerae, is carried by the genome of a filamentous phage, CTXPhi. Usually, specific CTXPhi infect each of the two important biotypes, classical and El Tor, of epidemic V. cholerae strains belonging to serogroup O1, and are called CTX(class)Phi and CTX(ET)Phi, respectively. However, an unusual hybrid El Tor strain carrying CTX(class)Phi caused the cholera epidemic in Mozambique in 2004. To understand the evolution of that strain, we have further analysed some representative hybrid El Tor strains isolated in Kolkata, India, in 1992, and the results indicate that both the Mozambique and the Indian strains are infected with a unique CTX(class)Phi having only four copies of the tandem heptamer repeat sequence 5'-TTTTGAT-3' present in the ctxAB promoter (P(ctxAB)) region, like in CTX(ET)Phi. Usually, the P(ctxAB) of the classical biotype contains seven to eight copies of such sequences. However, sequence analyses of the P(ctxAB) regions of several classical strains indicated that the copy number of heptamer repeat sequences might vary from four to eight copies, which was previously unknown. Since the hybrid strains analysed in this study carry four copies of the heptamer sequences, it may thus serve as a marker to trace the strain in future. Interestingly, while the Mozambique strain is devoid of an El Tor-specific free RS1 element or pre-CTX prophage, the Indian hybrid strains carry such elements. The free RS1 has been mapped, cloned and sequenced. As in pre-CTX and CTX prophages, multiple copies of free RS1 elements were found to be integrated in tandem in the large chromosomal dif site. Since Indian hybrid El Tor strains carry either free RS1 or pre-CTX prophage in their large chromosomes, it is possible that the Mozambique hybrid El Tor strain has evolved from these progenitor strains by step-wise deletion of CTX genetic elements from their large chromosomes.
ctxAB 操纵子在霍乱弧菌中编码霍乱毒素（CT），由丝状噬菌体 CTXPhi 的基因组携带。通常，特定的 CTXPhi 感染属于血清群 O1 的两种重要生物型，即经典型和 El Tor，分别称为 CTX(class)Phi 和 CTX(ET)Phi。然而，2004 年莫桑比克的霍乱流行是由携带 ctxAB 操纵子的非常规混合 El Tor 菌株引起的。为了了解该菌株的进化，我们进一步分析了 1992 年在印度加尔各答分离的一些代表性混合 El Tor 菌株，结果表明，莫桑比克和印度菌株均感染了一种独特的 CTX(class)Phi，其 ctxAB 启动子（P(ctxAB)) 区域中仅存在 4 个串联七聚体重复序列 5'-TTTTGAT-3'，与 CTX(ET)Phi 相同。通常，经典生物型的 P(ctxAB)含有 7 到 8 个这样的序列。然而，对几个经典菌株的 P(ctxAB)区域的序列分析表明，七聚体重复序列的拷贝数可能从 4 到 8 个不等，这是以前未知的。由于本研究分析的混合菌株携带 4 个七聚体序列，因此它可能成为未来追踪该菌株的一个标记。有趣的是，虽然莫桑比克菌株缺乏 El Tor 特异性的游离 RS1 元件或前 CTX 原噬菌体，但印度混合菌株携带这些元件。已对游离 RS1 进行了作图、克隆和测序。与前 CTX 和 CTX 原噬菌体一样，多个游离 RS1 元件串联整合在大染色体 dif 位点。由于印度混合 El Tor 菌株在其大染色体上携带游离 RS1 或前 CTX 原噬菌体，莫桑比克混合 El Tor 菌株可能是通过从其大染色体上逐步删除 CTX 遗传元件而从这些祖先生成菌株进化而来的。
