A Bacillus megaterium,? genomic fragment, which encoded an activator homologous to sigma(54) regulators and which was capable of activating Escherichia coli ato genes in trans was detected in a gene library of B. megaterium screened for beta-ketothiolase activity. The fragment presented only one complete open reading frame (ORF1), which encoded a protein of 398 amino acids. The recombinant plasmid complemented mutations in the Escherichia coil atoC regulatory gene. The constitutive expression of the E, coli aro operon mediated by ORF1 could be useful for the synthesis of polyhydroxyalkanoates with different flexibility properties by recombinant E, coli strains.