A Novel L-Serine Exporter and Its Positive Regulator in Corynebacterium glutamicum

Exporters play an essential role in the fermentative production of amino acids. In Corynebacterium glutamicum , ThrE, which can export L-threonine and L-serine, is the only identified L-serine exporter so far. In this study, a novel L-serine exporter NCgl0580 was identified and characterized in C. glutamicum ΔSSAAI (SSAAI), and named as SerE (encoded by serE). Deletion of serE in SSAAI led to a 56.5% decrease in L-serine titer, whereas overexpression of serE compensated for the lack of serE with respect to L-serine titer. A fusion protein with SerE and enhanced green fluorescent protein (EGFP) was constructed to confirm that SerE localized at the plasma membrane. The function of SerE was studied by peptide feeding approaches, and the results showed that SerE is a novel exporter for L-serine and L-threonine in C. glutamicum . Subsequently, the interaction of a known L-serine exporter ThrE and SerE was studied, and the results suggested that SerE is more important than ThrE in L-serine export in SSAAI. Probe plasmid and electrophoretic mobility shift assays (EMSA) revealed NCgl0581 as the transcriptional regulator of SerE. Comparative transcriptomics between SSAAI and the NCgl0581 deletion strain showed that NCgl0581 regulated the transcription of 115 genes in C. glutamicum , among which the transcriptional level of NCgl0580 decreased 280-fold in a NCgl0581 deletion strain, indicating that NCgl0581 is a positive regulator of SerE. Thus, this study provides a novel target for L-serine and L-threonine export engineering as well as a novel global transcriptional regulator NCgl0581 in C. glutamicum .