comB, a novel competence gene required for natural transformation of Acinetobacter sp BD413: identification, characterization, and analysis of growth-phase-dependent regulation

Here we describe five tandemly arranged and converging ORFs in Acinetobacte r sp. BD413, namely lytB, orfY, orfX, comB, and orfZ, located upstream of t he previously identified competence gene comC. The N-termini of the deduced proteins OrfY and ComB exhibit the conserved endopeptidase cleavage motifs Of prepilin proteins; the deduced protein ComB is similar to type IV pilin s. LytB is similar to the Escherichia toll LytB, which has been implicated in the stringent response. No homologues of OrfX, OrfY and OrfZ could be id entified. A mutation in orfY or orfZ led to 100-fold reduced transformation frequencies and a mutation in comB resulted in a noncompetent phenotype. D isruption of lytB did not affect the natural transformation phenotype. Comp lementation studies clearly demonstrated that comB is involved in natural t ransformation, whereas the transformation-deficient phenotypes of orfY and orfZ mutants were due to polar effects on comB and comC, respectively. Anal yses of the twitching motility phenotype and of the ultrastructure of the n oncompetent comB mutant suggested that the competence gene comB is not esse ntial for the biogenesis of type IV pill and expression of the type IV pill -associated property of twitching motility. Transcriptional fusions between comB and a promoter-fire lacZ gene were constructed, and analysis of growt h-phase-dependent transcription revealed increased expression of comB durin g prolonged exponential and stationary phases.