LuxS-independent formation of AI-2 from ribulose-5-phosphate

Buffered solutions of ribulose-5-phosphate, but not ribose-5-phosphate, were found to contain high levels of AI-2 activity following incubation at concentrations similar to those reported in vivo. To test whether this process contributes to AI-2 formation by bacterial cells in vivo, an improved Vibrio harveyi bioassay was used. In agreement with previous studies, culture supernatants of E. coli and Staphylococcus aureus luxS mutants were found not to contain detectable levels of AI-2 activity. However, low activities were detected in an E. coli pgi-eda-edd-luxS mutant, a strain which degrades glucose entirely via the oxidative pentose phosphate pathway, with ribulose-5-phosphate as an obligatory intermediate.Our results suggest that LuxS-independent formation of AI-2, via spontaneous conversion of ribulose-5-phosphate, may indeed occur in vivo. It does not contribute to AI-2 formation in wildtype E. coli and S. aureus under the conditions tested, but may be responsible for the AI-2-like activities reported for other organisms lacking the luxS gene.In the marine bacterium Vibrio harveyi, autoinducer 2 (AI-2) is one of three quorum-sensing molecules regulating the production of bioluminescence in a population-density-dependent fashion [1,2]. In recent years, numerous pathogenic and non-pathogenic bacteria have also been shown to produce AI-2 (for a recent review see [3]), and for this reason the molecule has been suggested to function in interspecies communication [1,4-7].AI-2 is the collective term for a group of signal molecules formed from a common precursor, 4,5-dihydroxy-2,3-pentanedione (DPD). DPD is generated by many bacteria as a by-product of the activated methyl cycle in a reaction catalysed by LuxS [4,8]. LuxS acts by cleaving S-ribosylhomocysteine (SRH) to yield homocysteine and the reactive DPD, which spontaneously cyclises to from a range of furanone derivatives (Fig. 1). Two of these, (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate (S-THMF-b