Improved production of poly-γ-glutamic acid with low molecular weight under high ferric ion concentration stress in Bacillus licheniformis ATCC 9945a

•High Fe3+ concentrations lead to high γ-PGA production and low molecular weight.•High Fe3+ concentrations increase carbon source utilization for γ-PGA production.•The genes ggt and cwlO lead to low molecular weight under high Fe3+ concentrations.

This study found that high ferric ion (Fe3+) concentrations lead to high poly-γ-glutamic acid (γ-PGA) production and low-molecular-weight γ-PGA. In cultures containing 7.4 mmol/L FeCl3, the average molecular weight of γ-PGA reached 3.18 ± 0.3 × 105 Da, which is a 76% decrease from that in the culture without FeCl3 (1.31 ± 0.14 × 106 Da). Cultures containing increasing concentrations of FeCl3 (0-7.4 mmol/L) showed a corresponding increase in γ-PGA yield from 26.57 ± 1.64 to 34.93 ± 1.05 g/L. Transcription level analysis indicated that high FeCl3 concentrations could enhance the expression of γ-PGA synthetase genes (pgsA, pgsB, and pgsC), which may contribute to the improved production of γ-PGA. In addition, γ-PGA degradation genes ggt (encoding γ-glutamyltransferase) and cwlO were found to contribute to a lower γ-PGA molecular weight in cultures containing 7.4 mmol/L FeCl3. The result of γ-glutamyltransferase activity was in agreement with the results of the transcription level analysis. Overall, this study would provide a novel method for the regulation of synthesis of γ-PGA with low molecular weight.

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