Figure 6. PagR binds two regions of the stm2344promoter. DNase 1 footprinting electropherograms from the coding strand
of Pstm2344 . Digestions were performed with 75
pmol PagR (black trace) and with no PagR (orange trace). This image is
representative of two independent experiments.
PagR is likely a member of the LacI/GalR family of
transcriptional regulators. Herein, we have presented data showing that
PagR regulates gene expression in a manner like other proteins that
belong to the LacI/GalR family of regulators. That is, we hypothesize
PagR binds an as-yet-unidentified co-effector, dimerizes and then binds
to the identified sequences within the region between pagR and
the tktDE operon with the concomitant repression of its own
expression, and the expression of adjacent genes encoding a putative
transport system and subunits of a metabolic enzyme (Fig. 7).
Figure 7. Working model of PagR regulation of the tktDEoperon. We hypothesize PagR requires a co-effector to dimerize, bind to
the identified sites within the intergenic region between pagRand the 5-gene operon whose distal genes encode the subunits of
transketolase C (TktC). Roman numerals represent: I, stm2344(putative PTS ascorbate transporter subunit IIA), II, stm2343(putative PTS mannitol subunit IIB), III, stm2342 (putative PTS
ascorbate transporter subunit IIC). No experimental evidence has been
reported for the putative transporter. The red polygon represents the
as-yet-unidentified PagR co-effector.
Moreover, PagR binding to the promoter region between its gene andstm2344 protects a palindromic DNA sequence (Fig. 6). The
repressor activity of PagR presented here contrasts with previous data
reported by others, who showed that PagR is an activator of theslyA gene (Jiang et al., 2020). This dual activity, however, is
not unprecedented for LacI/GalR family proteins (Swint-Kruse and
Matthews, 2009). It is currently unknown what causes the switch between
these functions for PagR. It is possible that the target of the
uncharacterized, putative phosphotransferase system (PTS)
(stm2342-4 ) adjacent to pagR may provide the signal, like
galactose derepressing GalR (Swint-Kruse and Matthews, 2009). Expression
of pagR increases under low phosphate and low magnesium
conditions (Jiang et al., 2020), and it could be that PagR is regulated
by phosphorylation or as a function of magnesium ions. All these
interesting questions are currently being investigated.