The molecular analysis of a genomic region of B. megaterium revealed the presence of a gene coding for the enzyme phosphotransbutyrylase (Ptb). The enzyme activity was measured throughout the different phases of growth in B. megaterium, and its activity was found to be maximal in the late exponential growth phase. The branched amino acids isoleucine and valine activated Ptb expression. Ptb(Bm) was capable of using butyryl-CoA and 2-methyl-propionyl CoA as substrates. Act(Bm), a sigma (54) regulator from B. megaterium whose gene is situated upstream from the ptb gene, activated its expression.