Lack of the Consensus Sequence Necessary for Tryptophan Prenylation in the ComX Pheromone Precursor

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ComX, an oligopeptide pheromone that stimulates the natural genetic competence controlled by quorum sensing in Bacillus subtilis and related bacilli, contains a prenyl-modified tryptophan residue. Since ComX is the only protein known to contain prenylated tryptophan, the universality of this unique posttranslational modification has yet to be determined. Recently, we developed a cell-free assay system in which the tryptophan residue in the ComX_RO-E-2 pheromone precursor derived from B. subtilis strain RO-E-2 can be geranylated by the ComQ_RO-E-2 enzyme. We report here our attempt to identify the consensus sequence surrounding the geranylated tryptophan residue by using the cell-free system with various ComX_RO-E-2 pheromone precursor analogs. We found that [47–58]ComX_RO-E-2, corresponding to the C-terminal 12-residue peptide of the pheromone precursor, contained a short sequence essential for geranylation. We also found that the length of the sequence between the tryptophan residue and the C-terminus was important for geranylation, and that some [47–58]ComX_RO-E-2 pheromone precursor amino acids were involved in the geranylation reaction. However, we could not identify a consensus sequence surrounding the geranylated tryptophan. Our evidence suggests that, like Rab which lacks a consensus sequence yet is geranylgeranyl-modified on a cysteine residue, the ComX pheromone and its precursor also lack a consensus sequence.