phaCAB

phaCAB is a gene cluster found in several bacteria, most notably Cupriavidus necator (formerly Ralstonia eutropha), that encodes the enzymes required for the biosynthesis of polyhydroxyalkanoates (PHAs), a class of intracellular biopolymers used as carbon and energy storage. The operon typically includes the three genes phaA, phaB, and phaC, whose coordinated expression enables the conversion of acetyl-CoA to polyhydroxyalkanoates such as polyhydroxybutyrate (PHB). PHAs accumulate as intracellular granules when there is an excess of carbon but limitation of other nutrients.

The enzymatic steps are as follows: phaA encodes β-ketothiolase, which condenses two acetyl-CoA molecules to form acetoacetyl-CoA; phaB encodes acetoacetyl-CoA reductase, which reduces acetoacetyl-CoA to 3-hydroxybutyryl-CoA using NADPH; and phaC encodes PHB synthase, which polymerizes 3-hydroxybutyryl-CoA into PHB. This pathway serves as a versatile platform for microbial production of biodegradable polyesters.

In biotechnology, the phaCAB operon is frequently expressed in heterologous hosts such as Escherichia coli or other bacterial systems to produce PHAs from renewable carbon sources. PHAs, including PHB, are of interest as biodegradable and biocompatible plastics, offering potential environmental advantages over conventional petrochemical polymers. Production yields depend on gene expression levels, host metabolism, and cultivation conditions, with strategies often including plasmid- or genome-based integration and nutrient-limiting, carbon-rich fermentation to promote polymer accumulation. Detection of PHB granules commonly uses staining methods such as Nile red.