Abstract
ABSTRACTAmino acid exporters play an important role in regulating amino acid production by Corynebacterium glutamicum, and over 90% of amino acid export is attributed to exporters in this species. ThrE was reported to be an l-serine exporter, and SerE was identified as an l-serine exporter in our previous study. However, when both ThrE and SerE were deleted, the l-serine titer was decreased by 60%, suggesting other l-serine exporters may exist. In the present study, NCgl0254 and NCgl0255 were identified as novel l-serine exporters through comparative transcriptomics and gene functional analyses. The contributions of the four exporters (ThrE, SerE, NCgl0254 and NCgl0255) in l-serine export were studied by gene deletion, gene overexpression, amino acid export assay and real-time quantitative PCR (RT-qPCR). The results showed that SerE is the major l-serine exporter in C. glutamicum. Fermentation and amino acid export assays of SSAAI, SSAAI-serE-thrE-ncgl0254-ncgl0255 and SSAAIΔserEΔthrEΔncgl0254Δncgl0255 indicated that the four l-serine exporters undertake most of the l-serine export, and their overexpression enhanced export of l-serine in SSAAI. When one l-serine exporter was deleted, the transcription level of the other three exporters was upregulated. However, the decrease in l-serine titer caused by deletion of one exporter was not fully compensated by upregulation of the other three exporters at the transcription level, indicating that l-serine production by C. glutamicum may be determined by cooperative efficiency of all four l-serine exporters, with each being interdependent.IMPORTANCEThis work identified the novel l-serine exporters NCgl0254 and NCgl0255, and revealed their roles in l-serine export alongside the l-serine exporters. All four l-serine exporters are interdependent and undertake most of the l-serine export, but SerE is the major l-serine exporter. The findings expand our knowledge of amino acid exporters in C. glutamicum, and the approach can be employed for exploring of bacterial exporters of unknown function.
Publisher
Cold Spring Harbor Laboratory