Phasin proteins expressed by Rhodopseudomonas palustris for bioplastic production

Polyhydroxyalkanoates (PHAs) are bioplastics produced by microbes that could replace conventional plastics. Phasins are a group of small proteins found in all PHA-producing organisms that have diverse functions for PHA metabolism including (i) activating PHA synthases and depolymerases, (ii) fostering compositional changes in PHA granules, and (iii) chaperone-like activities for cell fitness. Rhodopseudomonas palustris is a metabolically robust microbe that produces from lignin but its phasins have not been experimentally explored yet. Thus, the aim of this study is to employ combined transcriptomics and proteomics analyses to identify and characterize four predicted phasins in R. palustris' genome (RPA3770, RPA0089, RPA4137, and RPA4138). The gene expressions of the four bioinformatically-predicted phasins were significantly higher under PHA-producing conditions compared to non-PHA production. The only phasin gene to have a significantly higher fold change under mid-exponential growth was RPA3770, suggesting it may have a role in preventing PHA accumulation or perhaps a role in initiating PHA production. All four phasins proteins were detected under PHA-producing conditions by mass spectrometry. Based on the multi-omics analysis in this study, it is likely that RPA0089 and RPA3770 code for the dominant phasins employed by R. palustris. Ultimately, this study employs a targeted multi-omics approach to trailblaze the initial identification and characterization of phasin proteins that R. palustris uses for bioplastic production from lignin breakdown products. Future studies can build on this work to further progress R. palustris as an industrial powerhouse for sustainable production of bioplastics or perhaps use this information to "turn off" bioplastic production and funnel more energy toward other valuable bioproducts like biohydrogen.