Acyl-homoserine lactone quorum sensing in the Roseobacter clade

Abstract

Members of the Roseobacter clade are ecologically important and numerically abundant in coastal environments and can associate with marine invertebrates and nutrient-rich marine snow or organic particles, on which quorum sensing (QS) may play an important role. In this review, we summarize current research progress on roseobacterial acyl-homoserine lactone-based QS, particularly focusing on three relatively well-studied representatives, Phaeobacter inhibens DSM17395, the marine sponge symbiont Ruegeria sp. KLH11 and the dinoflagellate symbiont Dinoroseobacter shibae. Bioinformatic survey of luxI homologues revealed that over 80% of available roseobacterial genomes encode at least one luxI homologue, reflecting the significance of QS controlled regulatory pathways in adapting to the relevant marine environments. We also discuss several areas that warrant further investigation, including studies on the ecological role of these diverse QS pathways in natural environments.

Figure 1.

Basic model of quorum sensing (QS) circuits. The eclipse represents a cell. The I gene represents the luxI homologue. R represents the acylated homoserine lactone (AHL) receptor LuxR protein. The dark unfilled circle represents the LuxI enzyme while the dark solid dots represent the AHL molecules. Stalked arrows indicate the transcription of the genes and the dotted line with arrow shows the positive feedback by the complex of the LuxR receptor and AHLs on the AHL synthase gene. The solid line with arrow depicts the function of R complex on the target genes. Squiggly line indicates translation of I gene and solid curved line with arrow indicates enzymatic function of I gene. The left corner shows the basic structure of a typical AHL molecule. N can vary from 4 to 18. R can be oxo, OH or H.

Figure 2.

Interconnected QS network and model for ssaRI and cckA-chpT-ctrA regulatory circuit that controls KLH11 flagellar motility. Lengths of the ssaRI, ssbR, and sscI genes are drawn in scale. Genes and products are colored in corresponding colors, with R-genes more darkly colored and I-genes lighter colored. The dark blue dots represent the AHLs, mainly long chain (lc) 3-oxo-HSLs, synthesized by SsaI. The dark green dots represent the AHLs, mainly long chain (lc) 3-OH-HSLs, synthesized by SsbI. Lines with bars indicate inhibition and arrows indicate activation. Squiggly lines indicate translation of genes or products of enzyme action. The dashed line with arrows indicates the potential phosphate flow from CckA to CtrA via ChpT. The curved lines with arrows around CtrA indicate positive feedback loops. The “X” indicates the unknown regulator(s). The figure is modified from references [17] and [29].

Figure 3.

Structure of p-coumaroyl-HSL based on reference Schaefer et al. [52].