The Fis Nucleoid Protein Negatively Regulates the Phase Variation fimS Switch of the Type 1 Pilus Operon in Enteropathogenic Escherichia coli

Abstract

In <i>Escherichia coli</i> the expression of type 1 pili (T1P) is determined by the site-specific inversion of the <i>fimS</i> ON-OFF switch located immediately upstream of major fimbrial subunit gene <i>fimA</i>. Here we investigated the role of virulence (Ler, GrlR, and GrlA) and global regulators (H-NS, IHF, and Fis) in the regulation of the <i>fimS</i> switch in the human enteropathogenic <i>E. coli</i> (EPEC) O127:H6 strain E2348/69. This strain does not produce detectable T1P and PCR analysis of the <i>fimS</i> switch confirmed that it is locked in the OFF orientation. Among the regulator mutants analyzed, only the ∆<i>fis</i> mutant produced significantly high levels of T1P on its surface and yielded high titers of agglutination of guinea pig erythrocytes. Expression analysis of the <i>fimA</i>, <i>fimB</i>, and <i>fimE</i> promoters using <i>lacZ</i> transcriptional fusions indicated that only P<i>fimA</i> activity is enhanced in the absence of Fis. Collectively, these data demonstrate that Fis is a negative regulator of T1P expression in EPEC and suggest that it is required for the FimE-dependent inversion of the <i>fimS</i> switch from the ON-to-OFF direction. It is possible that a similar mechanism of T1P regulation exists in other intestinal and extra-intestinal pathogenic classes of <i>E. coli</i>.