Pseudomonas fluorescens WH6 secretes a germination-arrest factor (GAF) that we have
identified previously as 4-formylaminooxyvinylglycine. GAF irreversibly inhibits germination of the
seeds of numerous grassy weeds and selectively inhibits growth of the bacterial plant pathogen
Erwinia amylovora. WH6-3, a mutant that has lost the ability to produce GAF, contains a Tn5
insertion in prtR, a gene that has been described previously in some strains of P. fluorescens as
encoding a transmembrane regulator. As in these other pseudomonads, in WH6, prtR occurs
immediately downstream of prtI, which encodes a protein homologous to extracytoplasmic
function (ECF) sigma factors. These two genes have been proposed to function as a dicistronic
operon. In this study, we demonstrated that deletion of prtI in WT WH6 had no effect on GAF
production. However, deletion of prtI in the WH6-3 mutant overcame the effects of the Tn5
insertion in prtR and restored GAF production in the resulting double mutant. Complementation of
the double prtIR mutant with prtI suppressed GAF production. This overall pattern of prtIR
regulation was also observed for the activity of an AprX protease. Furthermore, reverse
transcription quantitative real-time PCR analysis demonstrated that alterations in GAF production
were mirrored by changes in the transcription of two putative GAF biosynthetic genes. Thus, we
concluded that PrtI exerted a negative regulatory effect on GAF production, although the
mechanism has not yet been determined. In addition, evidence was obtained that the transcription
of prtI and prtR in WH6 may be more complex than predicted by existing models