| Summary: | <p>Abstract</p> <p>Background</p> <p>In many bacteria, the signal molecule AI-2 is generated from its precursor <it>S</it>-ribosyl-L-homocysteine in a reaction catalysed by the enzyme LuxS. However, generation of AI-2-like activity has also been reported for organisms lacking the <it>luxS </it>gene and the existence of alternative pathways for AI-2 formation in <it>Escherichia coli </it>has recently been predicted by stochastic modelling. Here, we investigate the possibility that spontaneous conversion of ribulose-5-phosphate could be responsible for AI-2 generation in the absence of <it>luxS</it>.</p> <p>Results</p> <p>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 <it>in vivo</it>. To test whether this process contributes to AI-2 formation by bacterial cells <it>in vivo</it>, an improved <it>Vibrio harveyi </it>bioassay was used. In agreement with previous studies, culture supernatants of <it>E. coli </it>and <it>Staphylococcus aureus luxS </it>mutants were found not to contain detectable levels of AI-2 activity. However, low activities were detected in an <it>E. coli pgi-eda-edd-luxS </it>mutant, a strain which degrades glucose entirely via the oxidative pentose phosphate pathway, with ribulose-5-phosphate as an obligatory intermediate.</p> <p>Conclusion</p> <p>Our results suggest that LuxS-independent formation of AI-2, via spontaneous conversion of ribulose-5-phosphate, may indeed occur <it>in vivo</it>. It does not contribute to AI-2 formation in wildtype <it>E. coli </it>and <it>S. aureus </it>under the conditions tested, but may be responsible for the AI-2-like activities reported for other organisms lacking the <it>luxS </it>gene.</p>
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