ABSTRACT
Polyhydroxyalkanoates (PHA) have gained popularity as biodegradable polymers that could be used to displace some petroleum-derived plastics. PHA are polymerized by PHA synthase enzymes, which produce polymers that consist of either short-chain-length (scl-) or medium-chain-length (mcl-) subunits. PHA synthesis has been detected in heterotrophic, autotrophic, halophilic, and methylotrophic bacteria. Some bacteria, such as Cupriavidus necator and Pseudomonas spp., have revolutionized PHA production on the industrial scale. However, other bacterial species, such as Halomonas sp. and Paracoccus sp., have also been investigated as PHA producers. The availability of complete genomes for PHA producers has led to improved PHA production via genetic manipulation. Halomonas, with its ability to grow in high salt concentrations, has enabled low-cost PHA production processes in non-sterilized conditions. Paracoccus, a methylotrophic, denitrifying autotroph present in marine microbial mats, is also a producer of a high quantity of poly(3-hydroxybutyrtate) (PHB) under natural conditions. The enzymes that polymerize PHA in C. necator, Halomonas, and Paracoccus belong to the class I PHA synthases. These species synthesize and accumulate only short-chain-length PHA (scl-PHA) from different substrates. The enzymes that polymerize PHA in Pseudomonas species are grouped as class II PHA synthases and result in the synthesis and accumulation of medium-chain-length PHA with different monomer compositions depending on the carbon substrate used by the bacteria. Multiple PHA synthases (2–3) were identified in the respective genomes. All these PHA synthases have a lipase box and a catalytic domain possessing conserved active site residues Cys, His, and Asp. Genome analysis of PHA producers will identify new genes for manipulating and advancing PHA production.
