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

Abstract

Production of curli (AgF) adhesins by Escherichia coli and Salmonella enterica serovar Typhimurium (S. typhimurium) is associated with extracellular matrix production and is optimal at low temperature during stationary phase. Curli and extracellular matrix synthesis involves a complex regulatory network that is dependent on the CsgD (AgfD) regulator. We have identified a novel regulator, termed MlrA, that is required for curli production and extracellular matrix formation. Two cosmids from a genomic library of avian pathogenic E. coliχ7122 conferred mannose‐resistant haemagglutination (HA) and curli production to E. coli HB101, which is unable to produce curli owing to a defective regulatory pathway. The rpoS gene, encoding a known positive regulator of curli synthesis, and the E. coli open reading frame (ORF) of unknown function, yehV, identified on each of these cosmids, respectively, conferred curli 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 production by strain χ7122 were abolished by disruption of rpoS, mlrA or csgA, 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 curli production. An mlrA homologue was identified in S. typhimurium. Analysis of mlrA–lac operon fusions demonstrated that mlrA was positively regulated by rpoS. mlrA mutants of wild‐type S. typhimurium SL1344 or SR‐11 no longer produced curli or rugose colony morphology, and exhibited 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 curli production and aggregative morphology in an upregulated curli producing S. typhimurium derivative containing a temperature‐ and RpoS‐independent agfD promoter region. These results indicate that MlrA is a newly defined transcriptional regulator of csgD/agfD that acts as a positive regulator of RpoS‐dependent curli and extracellular matrix production by E. coli and S. typhimurium.

Comparative Genetics of the rdar Morphotype inSalmonella

“…Approximately 10,000 transformants were screened, and three rdar-positive isolates were obtained. Plasmids purified from all three isolates contained mlrA (yehV), a known positive regulator of agfD transcription (13,22). When Sarc1 was transformed with pBR322 containing only mlrA, this was sufficient to restore the rdar morphotype (Fig.…”

“…Mycobacterium bovis bacillus Calmette-Guérin (BCG) is one well-documented example of this phenomenon (6), which has also occurred in several commonly studied bacterial pathogens (21). The initial descriptions of curli (Tafi) (43) and the positive regulators crl (3) and mlrA (13) were for E. coli HB101, a commonly used strain with trans regulatory mutations. Garcia et al (22) have also recently described a variant of the commonly used S. enterica serovar Typhimurium SL1344 with a trans regulatory mutation.…”

“…The arrangement of OmpR binding sites in the agfD promoter region is similar to that in the well-characterized ompF promoter (33), with a high-affinity binding site for activation under conditions of low osmolarity and low-affinity binding sites that shut off transcription at higher osmolarity (23). The mechanism for MlrA activation of agfD transcription is unknown, and no binding sites have been identified (13). Phosphorylated CpxR (CpxR-P) acts as a repressor of agfD transcription in high salt concentrations, through binding to sites in both agfD and agfB promoter regions (35,46).…”

Differential Binding of Escherichia coli O157:H7 to Alfalfa, Human Epithelial Cells, and Plastic Is Mediated by a Variety of Surface Structures

“…The binding of K-12 strains to plant and mammalian cells was promoted by the addition of the cah, aidA1, or csg gene to the bacteria, suggesting that all of their gene products can participate in binding to both types of surfaces. While E. coli K-12 strains possess the genes for curli biosynthesis and a cah homologue gene (agn-43), it has been shown that the expression of their protein products is repressed or the genes are inactivated (6,17,21,45), which renders them unable to interfere with the phenotypes observed.…”

Evolutionary loss of the rdar morphotype in Salmonella as a result of high mutation rates during laboratory passage

“…agfD is also regulated by the transcriptional regulators OmpR, MlrA and CpxR-P (Gerstel and Rö mling, 2003;. s 38 not only directly regulates agfD, but also feeds into its activation via positive regulation of the transcriptional regulator mlrA, (Brown et al, 2001). Expression of agfD is initiated in late exponential phase with a peak in early stationary phase, which results in initiation of transcription of the rest of the fimbrial biosynthetic genes, and other genes in the regulon including adrA, (Rö mling et al, 2000) which is involved in regulating cellulose biosynthesis.…”