MlrA, a novel regulator of curli (AgF) and extracellular matrix synthesis by Escherichia coli and Salmonella enterica serovar Typhimurium

Production of curli (AgF) adhesins by Escherichia coli and Salmonella enter ica serovar Typhimurium (S. typhimurium) is associated with extracellular m atrix production and is optimal at low temperature during stationary phase. Curli and extracellular matrix synthesis involves a complex regulatory net work that is dependent on the CsgD (AgfD) regulator. We have identified a n ovel regulator, termed MlrA, that is required for curl! production and extr acellular matrix formation. Two cosmids from a genomic library of avian pat hogenic E. coli chi 7122 conferred mannose-resistant haemagglutination (HA) and curli production to E. coli HB101, which is unable to produce curli ow ing to a defective regulatory pathway. The rpoS gene, encoding a known posi tive regulator of curli synthesis, and the E. coli open reading frame (ORF) of unknown function, yehV, identified on each of these cosmids, respective ly, conferred curl! production and HA to E. coli HB101. We have designated yehV as the mlrA gene for MerR-like regulator A because its product shares similarities with regulatory proteins of the MerR family. HA and curli prod uction by strain chi 7122 were abolished by disruption of rpoS, mlrA or csg A, the curli subunit gene. Both csgD and csgBA transcription, required for expression of curli, were inactive in an mlrA mutant grown under conditions that promote curl! production. An mlrA homologue was identified in S. typh imurium. Analysis of mlrA-lac operon fusions demonstrated that mlrA was pos itively regulated by rpoS. mlrA mutants of wild-type S. typhimurium SL1344 or SR-11 no longer produced curli or rugose colony morphology, and exhibite d enhanced aggregation and extracellular matrix formation when complemented with the mlrA gene from either S. typhimurium or E. coli present on a low- copy-number plasmid. However, inactivation of mlrA did not affect curl! pro duction and aggregative morphology in an upregulated curli producing S. typ himurium, derivative containing a temperature- and RpoS-independent agfD pr omoter region. These results indicate that MlrA is a newly defined transcri ptional regulator of csgDlagfD that acts as a positive regulator of RpoS-de pendent curli and extracellular matrix production by E. coli and S. typhimu rium.