Resistance to serine in Bacillus subtilis: Identification of the serine transporter YbeC and of a metabolic network that links serine and threonine metabolism

Abstract

The Gram-positive bacterium Bacillus subtilis uses serine not only as building block for proteins but also as an important precursor in many anabolic reactions. Moreover, a lack of serine results in the initiation of biofilm formation. However, in excess serine inhibits the growth of B. subtilis. To unravel the underlying mechanisms, we isolated suppressor mutants that can tolerate toxic serine concentrations by three targeted and non-targeted genome-wide screens. All screens as well as genetic complementation in Escherichia coli identified the so far uncharacterized permease YbeC as the major serine transporter of B. subtilis. In addition to YbeC, the threonine transporters BcaP and YbxG make minor contributions to serine uptake. A strain lacking these three transporters was able to tolerate 100 mM serine whereas the wild type strain was already inhibited by 1 mM of the amino acid. The screen for serine-resistant mutants also identified mutations that result in increased serine degradation and in increased expression of threonine biosynthetic enzymes suggesting that serine toxicity results from interference with threonine biosynthesis. Originality-Significance StatementSerine is an important precursor for many biosynthetic reactions, and lack of this amino acid can induce biofilm formation in Bacillus subtilis. However, serine is toxic for the growth of B. subtilis. To understand the reason(s) for this toxicity and to identify the so far unknown serine transporter(s) of this bacterium, we performed exhaustive mutant screens to isolate serine-resistant mutants. This screen identified YbeC, the major serine transporter of B. subtilis. Moreover, we observed an intimate link between serine and threonine metabolism that is responsible for serine toxicity by inhibiting threonine biosynthesis.