Synthesis of poly (γ-glutamic acid) and heterologous expression of pgsBCA genes

The genes required for synthesis of poly (γ-glutamic acid) (γ-PGA) were cloned from Bacillus licheniformis NK-03, a strain isolated from fermented food, natto. There were three open reading frames pgsB, pgsC, pgsA in the cloned fragment, all of which were greatly similar with those from typical Bacillus subtilis strains. The alignment of deduced amino acid sequences showed that PgsC was the most conservative part in PgsBCA. Recombinant plasmid pXMJ19-PGS was constructed by a shuttle vector pXMJ19, and it was successfully transformed and expressed in the recombinant strains of Escherichia coli JM109 and Corynebacterium glutamicum ATCC13032, respectively. Expression of pgsBCA in C. glutamicum indicated that it could synthesize γ-PGA with a yield of 0.69 g/L and 97% proportion of l-glutamate monomer in the absence of glutamic acid. The results suggest that γ-PGA biosynthesis directly from glucose by genetic engineering is feasible and significant.