Introduction
Transketolases are well-characterized enzymes that catalyze the non-oxidative transfer of a C2 unit from a ketose to an aldose, shortening the ketose by two carbons and lengthening the aldose by two carbons (Schneider and Lindqvist, 1998). Transketolases use thiamin pyrophosphate (TPP) as cofactor, whose ylid carbanion triggers a nucleophilic attack on the carbonyl group of the ketose. The addition product cleaves in a retro-aldol like reaction and the resulting enamine adds to the carbonyl group of the aldose. Elimination of the TPP ylid carbanion yields the final ketose product. Transketolase-catalyzed reactions are critical in processes such as the non-oxidative branch of the pentose phosphate pathway (non-oxPPP) (Stincone et al., 2015) and the Calvin-Benson-Bassham (CBB) cycle (Raines, 2003). Up until recently, the genome of S. Typhimurium was thought to encode only two transketolases.
TktC was recently characterized as an isozyme of transketolase A and transketolase B (Shaw et al., 2018). The TktA and TktB transketolases are housekeeping enzymes involved in the non-oxPPP of this bacterium. The reversibility of reactions catalyzed by transketolase enzymes involved in the non-oxPPP allows the production of intermediate compounds to meet metabolic demands of the cell through the diversion of metabolites to other pathways such as replenishment of glycolysis, or to provide building blocks for the biosynthesis of aromatic amino acids, nucleotides, or lipopolysaccharides (Stincone et al., 2015). PagR is a transcriptional regulator that indirectly induces the expression of SPI-2 through slyA , which is important for S. Typhimurium replication in macrophages and pathogenicity in mice (Jiang et al., 2020). During the analysis of the genomic context of the pagRgene (locus tag stm2345 ), we noticed that pagR was transcribed divergently from a set of genes encoding a putative sugar transporter (stm2342 , stm2343 , stm2344 ) and two genes (tktD = stm2341 and tktE = stm2340 ) encoding the subunits of transketolase C (TktC, Fig. 1). In this paper we show that i) PagR regulates the expression of the divergently transcribedtktDE genes that encode the subunits of the TktC transketolase ofS. yphimurium; ii); PagR represses the expression of its own gene; and iii); PagR directly binds the promoter region betweenstm2344 and pagR , protecting two binding sites.