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.