Hobbs M, Dalrymple B P, Cox P T, Livingstone S P, Delaney S F, Mattick J S
Centre for Molecular Biology and Biotechnology, University of Queensland, Brisbane, Australia.
Mol Microbiol. 1991 Mar;5(3):543-60. doi: 10.1111/j.1365-2958.1991.tb00726.x.
The fimbrial subunit genes of Bacteroides nodosus may be divided into two distinct classes, based on the sequence of the major subunit gene fimA (accompanying paper--Mattick et al., 1991). The genetic organization of the fibrial gene region in these two classes is also distinct. Upstream of fimA in both classes in opposite transcriptional orientation is the gene aroA which encodes amino acid biosynthetic enzyme 5-enolpyruvylshikimate-3-phosphate synthase. However, downstream of fimA the two classes are quite different until homology is restored at a bidirectional transcription termination signal separating the fimbrial operon from a gene clpB, which appears to encode the regulatory subunit of an ATP-dependent protease. Between aroA and clpB class I strains contain, apart from fimA, only one other gene (fimB). Sequence and polymerase chain reaction analyses indicate that fimB does not have a separate promoter but rather is co-transcribed with fimA at a level attenuated by the strength of the transcription termination signal in the intergenic region. In class II strains fimA is followed by a more extended region containing three genes, which appear to have the same transcriptional arrangement as fimB. The second of these genes (fimD) may represent a functional analogue of fimB although there is no close sequence homology. The first gene (fimC) has no obvious similarity to either fimB or fimD. Beyond fimD, at the 3' end of the class II-specific region, is a variant fimbrial subunit gene (fimZ) which is virtually identical in serogroups D and H and which appears to represent a duplicate, possibly redundant, gene closely related to the progenitor of the more divergent structural subunit fimA gene found in these strains. Comparisons of the predicted fimZ product with those of fimA in class I and class II strains, as well as of the boundaries of the class-specific regions, suggest that the class II sequences evolved in another type 4 fimbriate species and were subsequently substituted in the B. nodosus genome by lateral transfer. Analysis of the sequences flanking fimA in different strains indicates that recombinational exchange of both fimA and the entire operon has also occurred between strains, and is possibly a mechanism for disseminating structural diversity in the population.
根据主要亚基基因fimA的序列（附文——马蒂克等人，1991年），结节拟杆菌的菌毛亚基基因可分为两个不同的类别。这两个类别的菌毛基因区域的遗传组织也有所不同。在这两个类别的fimA上游，以相反的转录方向存在aroA基因，该基因编码氨基酸生物合成酶5-烯醇丙酮酸莽草酸-3-磷酸合酶。然而，在fimA下游，这两个类别差异很大，直到在一个双向转录终止信号处恢复同源性，该信号将菌毛操纵子与一个clpB基因分开，clpB基因似乎编码一种ATP依赖性蛋白酶的调节亚基。在aroA和clpB之间，I类菌株除了fimA外，只包含另一个基因（fimB）。序列分析和聚合酶链反应分析表明，fimB没有独立的启动子，而是与fimA共转录，其水平因基因间区域转录终止信号的强度而减弱。在II类菌株中，fimA后面是一个更长的区域，包含三个基因，它们似乎与fimB具有相同的转录排列。这些基因中的第二个（fimD）可能代表fimB的功能类似物，尽管它们之间没有紧密的序列同源性。第一个基因（fimC）与fimB或fimD均无明显相似性。在fimD之后，在II类特异性区域 的3'端，是一个变异的菌毛亚基基因（fimZ），它在血清群D和H中几乎相同，似乎代表一个重复的、可能冗余的基因，与这些菌株中发现的差异更大的结构亚基fimA基因的祖先密切相关。将预测的fimZ产物与I类和II类菌株中的fimA产物以及类别特异性区域的边界进行比较，表明II类序列在另一种4型菌毛物种中进化，随后通过横向转移在结节拟杆菌基因组中被取代。对不同菌株中fimA侧翼序列的分析表明，fimA和整个操纵子在菌株之间也发生了重组交换，这可能是群体中结构多样性传播的一种机制。
