This study found that high ferric ion (Fe3+) concentrations lead to high poly-gamma-glutamic acid (gamma-PGA) production and low-molecular-weight gamma-PGA. In cultures containing 7.4 mmol/L FeCI3, the average molecular weight of gamma-PGA reached 3.18 +/- 0.3 x 10(5) Da, which is a 76% decrease from that in the culture without FeCl3 (1.31 +/- 0.14 x 10(6) Da). Cultures containing increasing concentrations of FeCl3 (0-7.4 mmol/L) showed a corresponding increase in gamma-PGA yield from 26.57 +/- 1.64 to 34.93 +/- 1.05 g/L. Transcription level analysis indicated that high FeCI3 concentrations could enhance the expression of gamma-PGA synthetase genes (pgsA, pgsB, and pgsC), which may contribute to the improved production of gamma-PGA. In addition, gamma-PGA degradation genes ggt (encoding gamma-glutamyltransferase) and cwlO were found to contribute to a lower gamma-PGA molecular weight in cultures containing 7.4 mmol/L FeCl3. The result of gamma-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 gamma-PGA with low molecular weight. (C) 2017 Elsevier Ltd. All rights reserved.