Regulation of bacterial surface attachment by a network of sensory transduction proteins

Abstract

Bacteria are often attached to surfaces in natural ecosystems. A surface-associated lifestyle can have advantages, but shifts in the physiochemical state of the environment may result in conditions in which attachment has a negative fitness impact. Therefore, bacteria employ numerous mechanisms to control the transition from an unattached to a sessile state. The Caulobacter crescentus protein HfiA is a potent developmental inhibitor of the secreted polysaccharide adhesin known as the holdfast, which enables permanent attachment to surfaces. Multiple environmental cues influence expression of hfiA , but mechanisms of hfiA regulation remain largely undefined. Through a forward genetic selection, we have discovered a multi-gene network encoding a suite of two-component system (TCS) proteins and transcription factors that coordinately control hfiA transcription, holdfast development and surface adhesion. The hybrid HWE-family histidine kinase, SkaH, is central among these regulators and forms heteromeric complexes with the kinases, LovK and SpdS. The response regulator SpdR indirectly inhibits hfiA expression by activating two XRE-family transcription factors that directly bind the hfiA promoter to repress its transcription. This study provides evidence for a model in which a consortium of environmental sensors and transcriptional regulators integrate environmental cues at the hfiA promoter to control the attachment decision.

Flagellar Perturbations Activate Adhesion through Two Distinct Pathways in Caulobacter crescentus

“…Transcription of hfiA is finely tuned by a complex hierarchy of transcription factors such that small changes in expression have significant impacts on holdfast production (33,35,37). The three nonadhesive mutants analyzed in Fig.…”

“…However, P hifA -lacZ activity differences for other key strains are modest and do not correlate perfectly with direct measurements of holdfast production. While cell cycle control and posttranscriptional processes can be masked in bulk reporter measurements, the expression level changes in flagellar signaling mutants are less pronounced than for regulatory systems that target hfiA directly (35,37). A significant portion of adhesion control exerted by the flagellum likely occurs independently of hfiA regulation.…”

“…Holdfast production is the primary determinant of surface colonization in C. crescentus, and its regulation is elaborate (26,33). In addition to cell cycle cues (7,34), nutrient availability (35), light (36), and redox status (37), mechanical contact has been implicated as an important activator of holdfast assembly (38). Recent evidence suggests that both flagella and T4P can stimulate holdfast production in response to contact with a surface (39,40), but conflicting models have emerged for how these transenvelope machines survey and disseminate mechanical information (34,41).…”

A cryptic transcription factor regulates Caulobacter adhesin development

“…Holdfast biogenesis is also influenced by mechanical cues [18-20], while the second messenger cyclic-di-GMP affects both synthesis [19] and physical properties of [21] the holdfast. Additionally, an elaborate regulatory pathway comprised of multiple two-component signaling (TCS) proteins and one-component regulators controls holdfast development and surface attachment [16]. We have previously shown that a C. crescentus strain expressing a non-phosphorylatable allele of the lovK sensor histidine kinase ( lovK H180A ) overproduces holdfast and, consequently, has an enhanced adhesion phenotype in a biofilm assay.…”

“…We have previously shown that a C. crescentus strain expressing a non-phosphorylatable allele of the lovK sensor histidine kinase ( lovK H180A ) overproduces holdfast and, consequently, has an enhanced adhesion phenotype in a biofilm assay. The lovK H180A adhesion phenotype requires the presence of spdS-spdR two-component system genes and the hybrid histidine kinase skaH gene [16]. Two XRE-family transcription factors, RtrA and RtrB, function downstream of the TCS regulators to promote holdfast synthesis by directly repressing transcription of the holdfast inhibitor, hfiA (Figure 1A).…”