Posttranscriptional regulation of PhbR, the transcriptional activator of polyhydroxybutyrate synthesis, by iron and the sRNA ArrF in Azotobacter vinelandii

Abstract

Azotobacter vinelandii is a Gram-negative bacterium able to synthesize poly-β-hydroxybutyrate (PHB), a biodegradable plastic of industrial interest. The phbBAC operon encodes the enzymes of PHB synthesis and is activated by the transcriptional regulator PhbR and the sigma factor RpoS. Iron limitation has been previously reported to increase PHB accumulation in A. vinelandii; however, the mechanism by which iron controls PHB synthesis is unknown. Under iron starvation in Escherichia coli, the RyhB sRNA modulates the translation of genes involved in iron homeostasis. ArrF is the RyhB analogue in A. vinelandii and similarly increases in quantity during Fe(2+) depletion. In this study, we evaluate the effect of iron and ArrF on PHB accumulation, and on phbR and phbBAC expression in A. vinelandii strain UW136. Using transcriptional and translational fusions of phbR and phbB with gusA reporter gene, we found that iron limitation increased the expression of phbBAC at the transcriptional level and posttranscriptionally increased the expression of phbR. We also found that the ArrF sRNA is a positive regulator of phbR expression at the posttranscriptional level. Collectively, these data suggest that iron limitation increases the translation of phbR through ArrF.