Kulasekara H D, Blomfield I C
Department of Microbiology and Immunology, Wake Forest University School of Medicine, Winston-Salem, NC 27157-1064, USA.
Mol Microbiol. 1999 Feb;31(4):1171-81. doi: 10.1046/j.1365-2958.1999.01257.x.
The expression of type 1 fimbriae in Escherichia coli is phase variable, with cells switching between fimbriate (ON) and afimbriate (OFF) phases. The phase variation is dependent on the orientation of a 314 bp DNA element (the switch) that undergoes DNA inversion. DNA inversion requires either fimB or fimE, site-specific recombinases that differ in both specificity and activity. Whereas fimB promotes recombination with little orientational bias, fimE promotes recombination in the ON-to-OFF direction exclusively. In wild-type cells, fimE activity predominates and, hence, most bacteria are afimbriate. Here, it is shown that fimE specificity is caused by two different, but complementary, mechanisms. First, FimE shows a strong preference for the switch in the ON orientation as a substrate for recombination. Differences in the nucleotide sequence of the recombinase binding sites is a key factor in determining FimE specificity, although one or more additional cis-active sites that flank the fim switch also appear to be involved. Secondly, the orientation of the switch controls fimE in cis, most probably to control recombinase expression.
大肠杆菌中1型菌毛的表达是相位可变的，细胞在菌毛形成（开启）和无菌毛（关闭）阶段之间转换。相位变化取决于一个314 bp的DNA元件（开关）的方向，该元件会发生DNA倒位。DNA倒位需要fimB或fimE，这两种位点特异性重组酶在特异性和活性上都有所不同。fimB促进重组时几乎没有方向偏好，而fimE仅促进从开启到关闭方向的重组。在野生型细胞中，fimE活性占主导，因此大多数细菌是无菌毛的。在此表明，fimE特异性由两种不同但互补的机制引起。首先，FimE强烈倾向于将处于开启方向的开关作为重组底物。重组酶结合位点的核苷酸序列差异是决定FimE特异性的关键因素，尽管fim开关侧翼的一个或多个额外顺式活性位点似乎也参与其中。其次，开关的方向顺式控制fimE，很可能是为了控制重组酶的表达。
