A genomic islet mediates flagellar phase variation in Escherichia coli strains carrying the flagellin-specifying locus flk.
The occurrence of unilateral flagellar phase variation was previously demonstrated in Escherichia coli strains carrying the non-fliC flagellin-specifying locus flk. In this study, we investigated the mechanism involved in this process. By using sequencing and sequence analysis, the flk region between the chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains, including the H35 strain identified in this study (the other strains used are H3, H36, H47, and H53 strains), and this region was found to contain a putative integrase gene and flanking direct repeats in addition to the flk flagellin-specifying gene flkA and a fliC repressor gene, flkB, indicating that there is a typical genomic islet (GI), which was designated the flk GI. The horizontal transfer potential of the flk GI was indicated by detection of the excised extrachromosomal circular form of the flk GI. By generating fliC-expressing variants of H3 and H47 strains, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and a complementation test. The potential integration sites of the flk GI were identified. A general model for flagellar phase variation in flk-positive E. coli strains can be expressed as fliC(off) + flkA(on) --> fliC(on) + flkA(none). This is the first time that a molecular mechanism for flagellar phase variation has been reported for E. coli.
Feng
Lu
L
TEDA School of Biological Sciences and Biotechnology, Nankai University, 23 Hongda Street, TEDA, Tianjin 300457, Peoples Republic of China. fenglu63@nankai.edu.cn
Liu
Bin
B
Liu
Yanqun
Y
Ratiner
Yuli A
YA
Hu
Bo
B
Li
Dan
D
Zong
Xiaolin
X
Xiong
Wei
W
Wang
Lei
L
D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000502:physiology; D029968:Escherichia coli Proteins / Q000235:genetics* / Q000502:physiology; D005407:Flagella / Q000502:physiology*; D005408:Flagellin / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial; D044404:Genomic Islands / Q000235:genetics*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA
D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D005408:Flagellin