Surface-sterilized wheat seeds were treated with bacterial cultures at 108 CFU mL−1 as described previously (Pierson et al., 1998). To determine bacterial colonization, sterile soil and natural soil (not autoclaved) were used to grow wheat seeds. After 7, 14 and 21 days, 10 plants were collected Trichostatin A supplier randomly from each treatment, and the population densities in the rhizosphere (1 g) and the root tip (1 cm) were determined as described by Hoben & Somasegaran (1982). The experiment was repeated twice, and population data collected as CFU counts were log10-transformed

before statistical analysis. Data were analyzed and compared by performing two-sample independent t-tests (P<0.05 was considered significant) using origin 7.0 software (Originlab Corporation). Strain 2P24 carrying a plasmid-borne phlA-lacZ fusion was subjected to a random mini-Tn5 insertion mutagenesis to identify novel regulators of the antibiotic 2,4-DAPG production. Among the ∼10 000 transposon mutants tested, one mutant designated PMphlA23, which exhibited the greatest reduction in phlA Omipalisib expression (83% decrease of the β-galactosidase activity compared with its parental strain), was selected and purified for further studies. The mini-Tn5-flanking

region in the PMphlA23 mutant was cloned, and sequence analysis revealed that the transposon was inserted into the upstream region between positions −16 and −17 of a locus that had 84% amino acid identity to Hfq (Fig. 1), encoding a key global regulator for stress resistance and virulence in Pseudomonas aeruginosa (Sonnleitner et al., 2003). A 3.2-kb BamHI fragment containing the entire hfq gene was cloned from the genomic DNA of strain 2P24 (see Materials and methods). Sequencing and blast analysis (Altschul et al., 1997) of this fragment revealed three ORFs (Fig. 1). The deduced

product (86 amino acids) of the hfq gene in strain 2P24 is very similar to the Hfq proteins of P. fluorescens Pf0-1 (accession number CP000094; 98% identity), Pseudomonas syringae pv. tomato DC3000 (accession number AAO58370; 95% identity), P. aeruginosa PAO1 (accession number AE004091; 84% identity) and E. coli O157:H7 (accession number AAG59368; 60% identity). Roflumilast As in P. aeruginosa PAO1 (Sonnleitner et al., 2003), the hfq gene in P. fluorescens 2P24 is localized between two ORFs, encoding a putative tRNA isopentenyltransferase (OrfA, 79% identity to gene PA4945 of P. aeruginosa PAO1) and a putative GTP-binding protein (OrfB, 83% identity to gene PA4943 of P. aeruginosa PAO1). This arrangement also appears to be conserved in other Pseudomonas spp. (data not shown). In order to determine the potential regulatory effect of the hfq gene on phlA expression, the p970Gm-phlA plasmid containing the phlA promoter fused to the lacZ (β-galactosidase) reporter gene was transformed into the hfq mutant PM107 and its parental strain 2P24.