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.