Natural transformation of Vibrio fischeri requires tfoX and tfoY

Abstract

Recent evidence has indicated that natural genetic transformation occurs in Vibrio cholerae, and that it requires both induction by chitin oligosaccharides, like chitohexaose, and expression of a putative regulatory gene designated tfoX. Using sequence and phylogenetic analyses we have found two tfoX paralogues in all sequenced genomes of the genus Vibrio. Like V. cholerae, when grown in chitohexaose, cells of V. fischeri are able to take up and incorporate exogenous DNA. Chitohexaose-independent transformation by V. fischeri was observed when tfoX was present in multicopy. The second tfoX paralogue, designated tfoY, is also required for efficient transformation in V. fischeri, but is not functionally identical to tfoX. Natural transformation of V. fischeri facilitates rapid transfer of mutations across strains, and provides a highly useful tool for experimental genetic manipulation in this species. The presence of chitin-induced competence in several vibrios highlights the potential for a conserved mechanism of genetic exchange across this family of environmentally important marine bacteria.

Figure 1

Two paralogous tfoX-like sequences exist in all fully-sequenced Vibrionaceae. Phylogenetic reconstruction of tfoX homologues found in 13 Vibrionaceae and two Gram-negative outgroups, rooted with Yersinia pestis KIM. The two shaded clades contain two groups of closely related tfoX-like sequence homologues found in all Vibrionaceae. One clade includes only previously identified tfoX-like sequence homologues (tfoX, light grey) while the other contains another, previously unidentified group of tfoX-like sequence homologues (tfoY, dark grey). Identical nodes obtained from three reconstruction methods (Bayesian, Maximum Likelihood [ML] and Maximum Parsimony [MP]) with support >50% are identified by three numbers: (top left) posterior probabilities from Bayesian analysis, multiplied by 100; (top right) bootstrap percentage from 500 likelihood pseudoreplicates; (bottom) bootstrap percentage from 1000 parsimony pseudoreplicates. The bar indicates 0.1 expected change per site; the asterisk (*) indicates that all methods give identical support values of 100.

Figure 2

Loci surrounding the tfoX paralogues of V. fischeri, V. cholerae, and V. vulnificus. Schematic representations of orthologous genomic regions of three Vibrio species. Solid black arrows indicate tfoX or tfoY. Grey arrows indicate ORFs found in the tfoX or tfoY loci of all completed Vibrio genomes. White arrows indicate ORF’s found in all completed Vibrio genomes except V. fischeri strains ES114 and MJ11. Unlined grey arrows indicate ORFs flanking the conserved tfoX or tfoY loci.