Release 7.5

The ComX pheromone

ComX, a pheromone involved in a major quorum sensing system in bacilli, is post-translationally modified by strain-specific prenylation.

In order to acquire genetic competence once the cell density has reached a critical threshold, bacteria have developed a sophisticated quorum-sensing system. This system proceeds through the release of a pheromone, comX, that activates a two-component system, which eventually propagates the signal into the cell. Both components of this system have been identified: they are the sensor histidine kinase, comP, and the response regulator, the transcription factor comA.

The crucial pheromone ComX is produced as an inactive precursor which is activated by 2 post-translational modifications (PTM): the prenylation of a conserved tryptophan residue and a proteolytic cleavage. The protein comQ is thought to catalyze the maturation of comX. ComX and comQ sequences show striking variability among different strains, as do the prenyl derivatives. The mass of the prenyl groups linked to comX has been determined by mass spectrometry. Surprisingly, three different masses were observed: 120Da, 136Da and 205Da, depending on the strain studied. The 136 and 205Da forms are thought to consist of farnesyl and geranyl groups, respectively. The structure of the 205Da prenyl group in strain W23/RO-E-2 was recently obtained (Q8VL79). It consists of a geranyl group bound to a cyclic tryptophan. Interestingly enough, the nature of the prenyl group was shown to depend on the comX sequence itself rather than on the origin of the modifying enzyme comQ.

To our knowledge, this is the first report of a post-translational prenylation catalyzed by a bacterial enzyme, despite the universal availability of the necessary isoprenoid substrates and the existence of various other lipid-modified proteins in this kingdom. The exact function of comX prenylation is not known, but, by analogy with the situation in eukaryotes, it may provide anchoring to membrane structures.

Of note, this PTM, like all other PTMs in UniProtKB/Swiss-Prot, is annotated using controlled vocabulary.