The N-acetyltransferase RimJ responds to environmental stimuli to repress pap fimbrial transcription in Escherichia coli.

A variety of environmental signals, including temperature, growth medium, carbon source, osmolarity, pH, oxygen level, and various ions, are known to regulate virulence gene expression in pathogenic bacteria. The expression of several genes is often coordinately regulated by one or more environmental cues (reviewed in references 15 and 36). Presumably, the bacterium uses these stimuli to determine whether it is within a host or, more specifically, to identify a particular environmental niche within the host. This regulation allows for a more efficient utilization of the bacterium's resources and may be necessary for productive colonization of the host. Pyelonephritis-associated pilus (Pap) expression is regulated by both phase variation and environmental regulatory mechanisms. In many strains of uropathogenic Escherichia coli, Pap expression allows the attachment of bacteria to uroepithelial cells, facilitating colonization of the upper urinary tract (41, 42). Phase variation enables individual bacteria within a given population to alternate between two states of expression: phase ON, in which they are expressing fimbriae, and phase OFF, in which they are not expressing fimbriae (33). Phase variation is controlled at the transcriptional level by the formation of specific DNA methylation patterns of two GATC sites, GATCprox and GATCdist, within the pap regulatory region (6, 9, 52). Formation of these patterns relies upon the global regulators deoxyadenosine methylase (Dam), leucine-responsive regulatory protein (Lrp), and the cyclic AMP (cAMP) receptor protein CAP, as well as the operon-specific proteins PapI and PapB (reviewed in references 30 and 52). We previously demonstrated that four environmental cues—low temperature, rich (Luria-Bertani [LB]) medium, glucose as a carbon source, and high osmolarity—decrease papBA transcription (6, 57, 60). These environmental cues control several E. coli fimbrial operons, confirming their importance in regulating virulence gene expression (18, 20, 21, 25, 29, 37, 39, 45, 46, 60). For the papBA operon, low temperature causes all cells to transition to a phase OFF state, both phenotypically and at the level of DNA methylation (7, 57). Glucose and high osmolarity decrease the rate at which cells transition into a phase ON state (7, 60). Two proteins are known to be important in the regulation of papBA transcription in response to environmental conditions, H-NS and RimJ. H-NS is a histone-like nucleoid structuring protein that binds to A-T-rich bent regions of DNA and regulates the expression of a number of environmentally controlled virulence genes (1, 51, 61). Under all growth conditions, papBA transcription is decreased, relative to a wild-type strain, in an hns651 mutant, indicating that H-NS plays a positive role in papBA transcription (54, 57, 60). However, the repression caused by environmental signals is either fully or partially relieved by an hns651 mutation such that transcription approximates levels measured for the mutant under activating conditions (57, 60). Under environmentally repressive conditions, H-NS inhibits the phase OFF→ON transition and can prevent methylation of the pap GATCprox and GATCdist sites at 23°C, but not 37°C (57). RimJ was initially identified in a thermoregulatory mutant screen in which random chromosomal mini-Tn10 (mTn10) insertions isolated within rimJ allowed papBA transcription at a low temperature (23°C) (58, 59). RimJ is the N-terminal acetyltransferase that modifies the ribosomal protein S5 (16). RimJ, unlike H-NS, is exclusively a negative regulator of papBA transcription: transcriptional levels in rimJ mutants are similar to levels measured in the wild-type strain grown under transcriptionally activating conditions (reference 59 and this study). The mechanism by which RimJ represses papBA transcription and how the modification of a ribosomal protein might be involved in this process are unknown. In this study, we provide evidence that RimJ controls papBA and papI transcription in response to multiple environmental cues and inhibits the phase OFF→ON transition. In the absence of RimJ, papBA transcription still relies upon the regulators necessary to establish a phase ON state (Lrp, cAMP-CAP, Dam, PapI, and PapB), while our analyses indicate that RimJ and H-NS work in separate pathways to control papBA transcription. RimJ appears to be a pap-specific regulator that does not control other fimbrial operons in response to environmental conditions. Additionally, our experiments indicate that the ability of RimJ to control transcription is environmentally regulated.