Elucidating Polyhydroxybutyrate metabolism in Streptomycetes

Question

Plastic pollution poses a significant environmental threat causing multiple problems harming life on earth. One potential approach to lessen plastic pollution is to replace conventional plastics with biodegradable plastics, such as polyhydroxybutyrate (PHB). PHB can be produced and broken down by various bacteria using PHB synthase (PhaC) and PHB depolymerase (PhaZ), making it an ideal candidate for a circular economy approach. The goal is to identify and characterise the enzymes involved in PHB metabolism in Streptomyces and to evaluate whether they can serve as biofactories for converting PHB into antibiotics, as they are known as prolific hydrolytic enzyme and antibiotic producers.

Methodology

S. venezuelae was utilised as a model strain and was tested in a growth assay for PHB degradation. Known PHB-metabolizing enzymes were used as queries to bioinformatically identify PHB-related proteins based on sequence or structural similarities. Knockout mutants were created for each of the putative candidates and were analysed phenotypically.

Results

S.venezuelae can grow in minimal media with PHB as the sole carbon source. We identified three potential enzymes involved in PHB metabolism in S. venezuelae: a putative PhaC enzyme (vnz_01345), an extracellular PHB depolymerase (PhaZ, vnz_35915), and a PHB dehydrogenase (BdhA, vnz_03225). Macrocolonies of the Δvnz_01345 strain (putative PhaC) and the Δvnz_03225 strain (putative BdhA) exhibited more difficulty in sporulation compared to the wild type (WT) when grown on high salt sporulation medium. In contrast, the strain Δvnz_35915 (putative PhaZ) demonstrated better growth than the WT under the same conditions.

Conclusion

We showed that S.venezuelae can utilize PHB as a carbon source and identified several candidate enzymes responsible for PHB metabolism in Streptomyces. The phenotypic analysis suggests that PHB may have a protective role under osmotic stress conditions, as it has been shown for other bacteria. Further characterisation is necessary to understand these enzymes better and to prove their role in PHB metabolism. However, these results are promising and indicate Streptomyces as a suitable PHB upcycling platform.