raiIR Genes Are Part of a Quorum-Sensing Network Controlled by cinI and cinR in Rhizobium leguminosarum

Abstract

Analysis of N-acyl-L-homoserine lactones (AHLs) produced by Rhizobium leguminosarum bv. viciae indicated that there may be a network of quorum-sensing regulatory systems producing multiple AHLs in this species. Using a strain lacking a symbiosis plasmid, which carries some of the quorum-sensing genes, we isolated mutations in two genes (raiI and raiR) that are required for production of AHLs. The raiIR genes are located adjacent to dad genes (involved in D-alanine catabolism) on a large indigenous plasmid. RaiR is predicted to be a typical LuxR-type quorum-sensing regulator and is required for raiI expression. The raiR gene was expressed at a low level, possibly from a constitutive promoter, and its expression was increased under the influence of the upstream raiI promoter. Using gene fusions and analysis of AHLs produced, we showed that expression of raiI is strongly reduced in strains carrying mutations in cinI or cinR, genes which determine a higher-level quorum-sensing system that is required for normal expression of raiIR. The product of CinI, N-(3-hydroxy-7-cis tetradecenoyl) homoserine lactone, can induce raiR-dependent raiI expression, although higher levels of expression are induced by other AHLs. Expression of raiI in a strain of Agrobacterium that makes no AHLs resulted in the identification of N-(3-hydroxyoctanoyl)-L-homoserine lactone (3OH,C 8 -HSL) as the major product of RaiI, although other AHLs that comigrate with N-hexanoyl-, N-heptanoyl-, and Noctanoyl-homoserine lactones were also made at low levels. The raiI gene was strongly induced by 3OH,C 8 -HSL (the product of RaiI) but could also be induced by other AHLs, suggesting that the raiI promoter can be activated by other quorum-sensing systems within a network of regulation which also involves AHLs determined by genes on the symbiotic plasmid. Thus, the raiIR and cinIR genes are part of a complex regulatory network that influences AHL biosynthesis in R. leguminosarum.

Quorum Sensing in Nitrogen-Fixing Rhizobia

“…R. leguminosarum RaiI and RaiR have 93 and 88% identity to R. etli CNPAF512 RaiI and RaiR, respectively (184). As expected, raiI is positively autoregulated by RaiR and 3-OH-C 8 -HSL but is also induced to a lesser extent by 3-OH-C 14:1 -HSL and 3-oxo-C 8 -HSL (184).…”

“…R. leguminosarum RaiI and RaiR have 93 and 88% identity to R. etli CNPAF512 RaiI and RaiR, respectively (184). As expected, raiI is positively autoregulated by RaiR and 3-OH-C 8 -HSL but is also induced to a lesser extent by 3-OH-C 14:1 -HSL and 3-oxo-C 8 -HSL (184). These observations help explain the effect of cinRI mutations on the production of some of the short-chain AHLs and also suggest that an additional 3-oxo-C 8 -HSL-producing locus may contribute to regulation of raiRI (184).…”

“…An additional plasmid (pIJ9001)-located locus, raiRI, has recently been identified (184); this gene synthesizes 3-OH-C 8 -HSL as its major product, with C 6 -HSL, C 7 -HSL, and C 8 -HSL as minor products (Table 2) (95). R. leguminosarum RaiI and RaiR have 93 and 88% identity to R. etli CNPAF512 RaiI and RaiR, respectively (184).…”

Potential Emergence of Multi-quorum Sensing Inhibitor Resistant (MQSIR) Bacteria

“…For nodulation efficiency, RhiI synthesized C6HSL, C7HSL and C8-HSL activate the rhiABC genes. These QSS also govern motility and exopolysaccharide production which is required for infection process, and biofilm formation [80,81]. In Sinorhizobium meliloti, sinI/ sinR and expR, regulate the Sin QSS through long chain AHLs: C12HSL to C18HSL [29,[82][83][84].…”

Future research trends in the major chemical language of bacteria

“…Functions required for symbiosis (Lithgow et al, 2000;Rodelas et al, 1999;Wilkinson et al, 2002;Wisniewski-Dye et al, 2002) Serratia liquefaciens SwrI/R C4-HSL Swarming motility …”