The ΔluxS strain producing CAI-1 expressed pcomEA-lux at levels n

The ΔluxS strain producing CAI-1 expressed pcomEA-lux at levels near WT, expression was further reduced for the ΔcqsA mutant that only produces AI-2, and the autoinducer-deficient mutant (ΔcqsAΔluxS) expressed comEA at levels similar to the QS-deficient ΔhapR mutant (Fig. 1). As expected, a ΔtfoX strain only activates comEA expression when

induced to express TfoX from the plasmid, and the absence of TfoX induction reduced comEA expression in all strains to levels ∼100 lower than the ΔhapR mutant (Fig. 2a, white bars). Thus, TfoX is required for comEA transcription, and CqsA and LuxS together enhance expression ∼50-fold relative to the ΔhapR mutant. CAI-1 is the major autoinducer and AI-2 is the minor autoinducer for comEA transcription, as reported for V. cholerae virulence factor production in vivo (Higgins et al., Akt inhibitor 2007; Duan & March, 2010). To quantify the contribution to DNA uptake of autoinducers produced by V. cholerae, we measured the transformation frequency of V. cholerae

WT, ΔhapR, and ΔluxO strains using a crab-shell microcosm system described previously GSK3235025 (Meibom et al., 2005). Transformation efficiency of WT and the ΔluxO mutant were maximal, and no transformants were detected with the ΔhapR mutant (Fig. 2b). The ΔluxS mutant, which produces CAI-1, was approximately fourfold impaired for transformation; however, both QS mutants (ΔcqsA and ΔcqsAΔluxS) that do not produce CAI-1 were severely compromised for transformation by ∼100-fold relative to WT (Fig. 2b). No transformants were obtained in the absence of extracellular TCL KanR DNA, or when extracellular DNA was unmarked (data not shown), and in ΔtfoX (Fig. 2b), and ΔcomEA mutants, as described previously (Meibom et al., 2005). Thus, autoinducers produced by V. cholerae within a single-species biofilm promote DNA uptake. The discrepancy between the transformation frequency of the ΔcqsAΔluxS and the ΔhapR mutants may reflect that QS sRNAs constitutively expressed in the autoinducer-deficient strain do not completely eliminate all

hapR mRNA (Bardill et al., 2011). Apparently, low levels of HapR protein can occasionally promote DNA uptake in this 24-h assay where rare transformation events may be amplified by replication. Alternatively, it is possible that the presence of chitin used for transformation measurements (Fig. 2b) may provide signaling information, in addition to CAI-1 and AI-2, that is different from conditions when comEA expression is measured without chitin in the presence of TfoX induction (Fig. 2a). To test directly the role of autoinducers in comEA transcription and DNA uptake, purified CAI-1 and AI-2 where applied to the V. cholerae autoinducer-deficient ΔcqsAΔluxS mutant under the conditions described above. As shown for the V. cholerae autoinducer synthase mutants (Fig. 2a), the presence of both purified autoinducers (at saturating concentrations of 10 μM) resulted in maximal comEA expression by the autoinducer-deficient V.

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