L-carnitine is a crucial quaternary ammonium compound widely used in the pharmaceutical, food, and feed industries. Microbial biosynthesis of L-carnitine, compared with chemical synthesis, offers milder conditions, higher stereoselectivity, and a lower environmental impact. However, highly efficient strains and mechanistic insights into the bioconversion of γ-butyrobetaine (γBB) to L-carnitine remain limited. This study focuses on strain WQ-1, a newly screened strain capable of converting γBB to L-carnitine. Based on morphological, physiological, and phylogenetic analyses of 16S rRNA and housekeeping genes, the strain was identified as Ensifer sp. WQ-1. Under the condition of 30 °C, initial pH 8.5, 10% inoculum, 6 g/L initial γBB, shake-flask fermentation reached molar conversion rate of 88%. In a 5 L bioreactor fed-batch fermentation, the L-carnitine titer achieved 13.98 g/L with a 78.7% molar conversion rate. Genomic analysis revealed a 6.97 Mb genome harboring 6568 protein-coding genes, including candidates for quaternary ammonium transport, CoA-dependent transformation, and transcriptional regulation. Comparative transcriptomics identified 58 differentially expressed genes, highlighting the significant upregulation of genes related to acyl-CoA activation, dehydrogenation, carnitine metabolism, and thioester hydrolysis in the presence of γBB. Multi-omics analyses support a putative CoA-dependent metabolic pathway for conversion of γBB to L-carnitine in Ensifer sp. WQ-1.
Wei et al. (Fri,) studied this question.