Identification of proteins that transport SeMCys and the SeMet synthesis caused by their knockout

Seleno-amino acids are important organic selenium supplements with anti-cancer potential. Through metabolic engineering, Bacillus subtilis can be utilized to synthesize and secrete Se -methylselenocysteine (SeMCys). Transporters have a significant impact on amino acid metabolism, but the proteins that capable to transport SeMCys are unclear. This study screened the differentially expressed transporters through transcriptome analysis of B. subtilis during SeMCys fermentation. Knockout of the transporter encoding genes ycgH , ywoD , and yrdR led to a reduction of more than 50% in SeMCys uptake, and genetic complementation restored the uptake capacity. In addition, knockout of these transporter genes decreased SeMCys yield by at least one quarter while raising intracellular SeMCys levels. Individual overexpression of the three transporters improved SeMCys production. Among them, yrdR overexpression brought about a 53.3% rise in titer. Upon transporter genes knockout, selenomethionine (SeMet) emerged as a new product during SeMCys fermentation. Genes belonging to the SeMet biosynthetic pathway exhibited elevated transcription levels, whereas metK responsible for SeMet consumption and mccB that facilitates selenocysteine biosynthesis were down regulated. This study delivers novel insights into amino acid transportation and selenium metabolism.