A genomic islet mediates flagellar phase variation in Escherichia coli strains carrying flagellin-specifying locus flk

TEDA School of Biological Sciences and Biotechnology, Nankai University; Tianjin Key Laboratory of Microbial Functional Genomics; Tianjin Research Center for Functional Genomics and Biochip; The Engineering and Research Center for Microbial Functional Genomics and Detection Technology, Ministry of Education, and The Key Laboratory of Molecular Microbiology and Technology, Ministry of Education; 23 Hongda Street, TEDA, Tianjin 300457, P. R. China, Department of Microbiology, Mechnikov Research Institute for Vaccines and Sera, The Russian Academy of Medical Sciences, Moscow 105064, Russia

^* To whom correspondence should be addressed. Email: fenglu63{at}nankai.edu.cn. wanglei{at}nankai.edu.cn.

	Abstract

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 sequencing and sequence analysis, the flk region between chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains including H35 identified in this study (the others are H3, H36, H47, and H53), and found to contain a putative integrase gene and flanking direct repeats in addition to the flk-specifying flagellin gene flkA and a fliC repressor gene flkB, indicating a typical genomic islet, named the flk GI. The horizontal transfer potential of the flk GI was indicated by the detection of the excised extrachromosomal circular form of the flk GI. By generating the fliC expressing variants of H3 and H47, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by the excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and 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 shown as fliC^off + flkA^on fliC^on + flkA^-. This is the first time that a molecular mechanism for flagellar phase variation has been reported in E. coli.