A SITE-SPECIFIC RECOMBINASE IS REQUIRED FOR COMPETITIVE ROOT COLONIZATION BY PSEUDOMONAS-FLUORESCENS WCS365

A colonization mutant of the efficient root-colonizing biocontrol stra in Pseudomonas fluorescens WCS365 is described that is impaired in com petitive root-tip colonization of gnotobiotically grown potato, radish , wheat, and tomato, indicating a broad host range mutation. The colon ization of the mutant is also impaired when studied in potting soil, s uggesting that the defective gene also plays a role under more natural conditions, A DNA fragment that is able to complement the mutation fo r colonization revealed a multicistronic transcription unit composed o f at least six ORFs with similarity to lppL, lysA, dapF, orf235/233, x erC/sss, and the largely incomplete orf238, The transposon insertion i n PCL1233 appeared to be present in the orf235/233 homologue, designat ed orf240, Introduction of a mutation in the xerC/sss homologue reveal ed that the xerC/sss gene homologue rather than orf240 is crucial for colonization, xerC in Escherichia coli and sss in Pseudomonas aerugino sa encode proteins that belong to the lambda integrase family of site- specific recombinases, which play a role in phase variation caused by DNA rearrangements, The function of the xerC/sss homologue in coloniza tion is discussed in terms of genetic rearrangements involved in the g eneration of different phenotypes, thereby allowing a bacterial popula tion to occupy various habitats, Mutant PCL1233 is assumed to be locke d in a phenotype that is not well suited to compete for colonization i n the rhizosphere, Thus we show the importance of phase variation in m icrobe-plant interactions.