Signalling by the fungus Pythium ultimum represses expression of two ribosomal RNA operons with key roles in the rhizosphere ecology of Pseudomonas fluorescens F113

Pseudomonas fluorescens F113 produces antifungal metabolites that protect t he roots of sugarbeet from the fungus Pythium ultimum, The phytopathogen, i n turn, has the ability to downregulate the expression of genes fundamental to the rhizosphere competence of the bacterial strain. This paper describe s the characterization of two of these genes, which were isolated by screen ing a mini-Tn5:: lacZ mutant bank for differential expression of beta-galac tosidase in the presence of P. ultimum. In order to identify the genes affe cted in reporter mutants SF3 and SF5, the transposons and flanking regions were cloned. Sequence analysis of the regions flanking the transposons in S F3 revealed that mini-Tn5::lacZ had inserted into a tRNA(IIe) gene, which m aps within a ribosomal RNA (rrn) operon, In SF5, the transposon inserted be tween the promoter of a second rm operon and a gene encoding a 16S rRNA. So uthern blot analysis demonstrated that there are five rrn operons in P, flu orescens F113 and that the transposons in SF3 and SF5 had inserted into two different operons, Further characterization of these mutants suggests that their reduced rhizosphere competence is not the result of reduced viabilit y in the short term but may be accounted for partly by reduced growth rates under conditions that support rapid growth. Analysis of lacZ expression in the reporter mutants indicate that the marked rm operons are regulated dif ferently, suggesting different physiological roles.