Multiple N -Acyl Homoserine Lactone Signals of Rhizobium leguminosarum Are Synthesized in a Distinct Temporal Pattern

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Abstract
A common form of bacterial quorum sensing involves the production and release of acyl homoserine lactone (AHL) signal metabolites. The nitrogen-fixing symbiont Rhizobium leguminosarum reportedly produces at least six different AHLs, but little is known about the regulation of biosynthesis of these molecules. We used a radiolabeling protocol to quantify the relative amounts of AHLs synthesized over time by R. leguminosarum cells with and without the symbiosis plasmid pRL1JI. Cells containing pRL1JI were found to produce three predominant signals. In decreasing order of abundance, these were N -(3-oxo)octanoyl homoserine lactone [(3-O)C 8 HSL], N -octanoyl homoserine lactone, and N -hexanoyl homoserine lactone. Cells without pRL1JI produced only two major signals, N -(3-hydroxy-7- cis )tetradecanoyl homoserine lactone [(3-OH)C 14:1 HSL] and (3-O)C 8 HSL. Each AHL exhibited a distinct temporal pattern of synthesis, suggesting that each AHL is subject to unique regulatory mechanisms. While (3-O)C 8 HSL was produced in both cultures, the patterns of synthesis were different in cells with and without pRL1JI, possibly as a result of redundant gene functions that are present on both the chromosome and the symbiosis plasmid. None of the AHLs appeared to regulate its own biosynthesis, although exogenous (3-OH)C 14:1 HSL did activate synthesis of the three AHLs made by cells containing pRL1JI. These results indicate that the synthesis of multiple AHLs in R. leguminosarum is regulated by complex mechanisms that operate independently of quorum sensing itself but that (3-OH)C 14:1 HSL can supersede these controls in pRL1JI-containing cells. This work provides an important global perspective for AHL regulation that both complements and contrasts with the results of previous studies performed with isolated gene systems.