Role of Vfr in regulating exotoxin A production by Pseudomonas aeruginosa

Pseudomonas aeruginosaexotoxin A (ETA) production depends on the virulence-factor regulator Vfr. Recent evidence indicates that theP. aeruginosairon-starvation sigma factor PvdS also enhances ETA production through the ETA-regulatory generegA. Mutants defective invfr,regAandpvdS, plasmids that overexpress these genes individually andlacZtranscriptional/translational fusion plasmids were utilized to examine the relationship betweenvfr,regAandpvdSin regulatingP. aeruginosaETA production. ETA concentration andregAexpression were reduced significantly in PAOΔvfr, butpvdSexpression was not affected. Overexpression of Vfr produced a limited increase in ETA production in PAOΔpvdS, but not PAOΔregA. Additionally, overexpression of either RegA or PvdS did not enhance ETA production in PAOΔvfr. RT-PCR analysis showed that iron did not affect the accumulation ofvfrmRNA in PAO1. These results suggest that: (i) Vfr enhancestoxAexpression in PAO1 both directly and indirectly throughregA, but not throughpvdS; (ii)vfrexpression is not regulated by iron; and (iii) both Vfr and PvdS cooperate in the presence of RegA to achieve a maximum level oftoxAexpression.