Catalyzes the conversion of acetoacetate into acetoacetyl-CoA. Is involved in poly-3-hydroxybutyrate (PHB) degradation, which allows growth of R.meliloti on PHB cycle intermediates. acetoacetate + ATP + CoA = acetoacetyl-CoA + AMP + diphosphate Biopolymer metabolism; poly-(R)-3-hydroxybutanoate degradation. Acetylated. Deacetylation by the SIR2-homolog deacetylase activates the enzyme. Acetyl-CoA synthetase activity is not affected in mutant cells, whereas acetoacetyl-CoA synthetase activity is drastically reduced. Moreover, after growth in YM medium, cells lacking this gene accumulate PHB to 60 to 70% of cell dry mass. They display reduced ability to proliferate during the first 30 days of incubation in growth medium lacking nutrient carbon. Belongs to the ATP-dependent AMP-binding enzyme family.