A site-specific recombinase is required for competitive root colonization by Pseudomonas fluorescens WCS365.
A colonization mutant of the efficient root-colonizing biocontrol strain Pseudomonas fluorescens WCS365 is described that is impaired in competitive root-tip colonization of gnotobiotically grown potato, radish, wheat, and tomato, indicating a broad host range mutation. The colonization of the mutant is also impaired when studied in potting soil, suggesting that the defective gene also plays a role under more natural conditions. A DNA fragment that is able to complement the mutation for colonization revealed a multicistronic transcription unit composed of at least six ORFs with similarity to lppL, lysA, dapF, orf235/233, xerC/sss, and the largely incomplete orf238. The transposon insertion in PCL1233 appeared to be present in the orf235/233 homologue, designated orf240. Introduction of a mutation in the xerC/sss homologue revealed that the xerC/sss gene homologue rather than orf240 is crucial for colonization. xerC in Escherichia coli and sss in Pseudomonas aeruginosa 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 colonization is discussed in terms of genetic rearrangements involved in the generation of different phenotypes, thereby allowing a bacterial population to occupy various habitats. Mutant PCL1233 is assumed to be locked in a phenotype that is not well suited to compete for colonization in the rhizosphere. Thus we show the importance of phase variation in microbe-plant interactions.
Dekkers
L C
LC
Leiden University, Institute of Molecular Plant Sciences, Clusius Laboratory, Wassenaarseweg 64, 2333AL Leiden, The Netherlands.
Phoelich
C C
CC
van der Fits
L
L
Lugtenberg
B J
BJ
D000595:Amino Acid Sequence; D004254:DNA Nucleotidyltransferases / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics*; D029968:Escherichia coli Proteins; D019426:Integrases; D008969:Molecular Sequence Data; D009154:Mutation; D010944:Plants / Q000382:microbiology; D010957:Plasmids; D011551:Pseudomonas fluorescens / Q000235:genetics* / Q000254:growth & development; D045522:Recombinases; D016415:Sequence Alignment
D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D045522:Recombinases; C473360:XerC protein, E coli; D004254:DNA Nucleotidyltransferases; D019426:Integrases; C115006:integron integrase IntI1