Effects of MreB paralogs on poly-γ -glutamic acid synthesis and cell morphology in Bacillus amyloliquefaciens

Abstract

Actin-like MreB paralogs play important roles in cell shape maintenance, cell wall synthesis and the regulation of the D,L-endopeptidases, CwlO and LytE. The gram-positive bacteria, Bacillus amyloliquefaciens LL3, is a poly-γ -glutamic acid (γ -PGA) producing strain that contains three MreB paralogs: MreB, Mbl and MreBH. In B. amyloliquefaciens, CwlO and LytE can degrade γ -PGA. In this study, we aimed to test the hypothesis that modulating transcript levels of MreB paralogs would alter the synthesis and degradation of γ -PGA. The results showed that overexpression or inhibition of MreB, Mbl or MreBH had distinct effects on cell morphology and the molecular weight of the γ -PGA products. In fermentation medium, cells of mreB inhibition mutant were 50.2% longer than LL3, and the γ -PGA titer increased by 55.7%. However, changing the expression level of mbl showed only slight effects on the morphology, γ -PGA molecular weight and titer. In the mreBH inhibition mutant, γ -PGA production and its molecular weight increased by 56.7% and 19.4%, respectively. These results confirmed our hypothesis that suppressing the expression of MreB paralogs might reduce γ -PGA degradation, and that improving the cell size could strengthen γ -PGA synthesis. This is the first report of enhanced γ -PGA production via suppression of actin-like MreB paralogs.