Engineering Expression Cassette of pgdS for Efficient Production of Poly-γ-Glutamic Acids With Specific Molecular Weights in Bacillus licheniformis

Poly-gamma-glutamic acid (gamma-PGA) is an emerging biopolymer with various applications and gamma-PGAs with different molecular weights exhibit distinctive properties. However, studies on the controllable molecular weights of biopolymers are limited. The purpose of this study is to achieve production of gamma-PGAs with a wide range of molecular weights through manipulating the expression of gamma-PGA depolymerase (PgdS) in Bacillus licheniformis WX-02. Firstly, the expression and secretion of PgdS were regulated through engineering its expression elements (four promoters and eight signal peptides), which generated gamma-PGAs with molecular weights ranging from 6.82 x 10(4) to 1.78 x 10(6) Da. Subsequently, through combination of promoters with signal peptides, the production of gamma-PGAs with a specific molecular weight could be efficiently obtained. Interestingly, the gamma-PGA yield increased with the reduced molecular weight in flask cultures (Pearson correlation coefficient of -0.968, P<.01). Finally, in batch fermentation, the highest yield of gamma-PGA with a weight-average molecular weight of 7.80 x 10(4) Da reached 39.13 g/L under glutamate-free medium. Collectively, we developed an efficient strategy for one-step production of gamma-PGAs with specific molecular weights, which have potential application for industrial production of desirable gamma-PGAs.