A cryptic transcription factor regulates Caulobacter adhesin development

Abstract

Alphaproteobacteria commonly produce an adhesin that is anchored to the exterior of the envelope at one cell pole. In Caulobacter crescentus this adhesin, known as the holdfast, facilitates attachment to solid surfaces and cell partitioning to air-liquid interfaces. An ensemble of two-component signal transduction (TCS) proteins controls C. crescentus holdfast biogenesis by indirectly regulating expression of HfiA, a potent inhibitor of holdfast synthesis. We performed a genetic selection to discover direct hfiA regulators that function downstream of the adhesion TCS system and identified rtrC, a hypothetical gene. rtrC transcription is directly activated by the adhesion TCS regulator, SpdR. Though its primary structure bears no resemblance to any defined protein family, RtrC binds and regulates dozens of sites on the C. crescentus chromosome via a pseudo-palindromic sequence. Among these binding sites is the hfiA promoter, where RtrC functions to directly repress transcription and thereby activate holdfast development. Either RtrC or SpdR can directly activate transcription of a second hfiA repressor, rtrB. Thus, environmental regulation of hfiA transcription by the adhesion TCS system is subject to control by an OR-gated type I coherent feedforward loop; these regulatory motifs are known to buffer gene expression against fluctuations in regulating signals. We have further assessed the functional role of rtrC in holdfast-dependent processes, including surface adherence to a cellulosic substrate and formation of pellicle biofilms at air-liquid interfaces. Strains harboring insertional mutations in rtrC have a diminished adhesion profile in a competitive cheesecloth binding assay and a reduced capacity to colonize pellicle biofilms in select media conditions. Our results add to an emerging understanding of the regulatory topology and molecular components of a complex bacterial cell adhesion control system.

Sticky decisions: The multilayered regulation of adhesin production by bacteria

“…For these reasons, adhesion is usually the result of combined actions of several tightly regulated processes. A recent PLOS Genetics paper by Maeve McLaughlin and colleagues describes a new transcription factor involved in the regulation of bacterial adhesion and sheds light on the complexity and multifactorial nature of this regulation [ 1 ].…”

“…The work by McLaughlin and colleagues highlights a new transcription factor involved in this pathway, RtrC. The latter binds a pseudo-palindromic motif present in the hfiA promoter region and represses its expression [ 1 ]. All the work accumulated by the Crosson laboratory over the years places HfiA as a key player in holdfast regulation ( Fig 1 ).…”

“…In addition, while RtrA and RtrB are widely spread in the Alphaproteobacteria, RtrC homologs can only be found in Caulobacterales closely related to C . crescentus [ 1 ] ( Fig 2 ). But while HfiA is unique to C .…”

“…In C . crescentus , SpdS / SpdR controls the expression of at least three hfiA transcriptional regulators: RtrA, RtrB [ 17 ], and the newly described RtrC [ 1 ]. These regulators modulate holdfast production and biofilm formation.…”

“…shows that, in C . crescentus , RtrC can act as a repressor or activator of gene expression depending on its binding site location [ 1 ]. In addition to the hfiA promoter, RtrC binds to the promoter of genes involved in cdG signaling, motility, and chemotaxis [ 1 ].…”