Outer membrane protein I is associated with poly-β-hydroxybutyrate granules and is necessary for optimal polymer accumulation in Azotobacter vinelandii on solid medium

Azotobacter vinelandii is a soil bacterium that is able to synthesize poly-β-hydroxybutyrate (PHB), a polymer used to produce biodegradable plastic. PHB is stored in the cytoplasm as granules surrounded by several proteins such as the major phasin PhbP, PHB synthase and PHB depolymerase, among others. Many studies have reported the presence of membrane proteins on PHB granules due to contamination during the polymer extraction procedures. Previously, the outer membrane protein I (OprI) was detected on the polymer granules in A. vinelandii . In this study, by using random transposon mutagenesis, we identified that a mutation in the oprI gene diminished PHB accumulation in A. vinelandii on solid medium. Electron microscopy confirmed the low polymer production by the oprI mutant. Analysis of PHB granules by Tricine-SDS-PAGE revealed that the absence of OprI affected the protein profile of the granules, suggesting that OprI could have a structural role in A. vinelandii . Thus, some membrane proteins on PHB granules may be not be artefacts as previously described.