Fig. 6: Model of Pulcherrimin Regulation by ScoC, AbrB, and PchR.

A) Pulcherriminic acid biosynthesis by the cyclization of tRNA-charged leucines to form cyclo(L-leucine-leucine) and the subsequent oxidation by CypX to form water-soluble pulcherriminic acid. Pulcherriminic acid is then transported out of the cell by YvmA, where it can form the insoluble pulcherrimin complex with iron, which forms a red color and has a peak absorbance at 410 nm. B) During exponential growth, PchR, AbrB, and ScoC bind directly to the yvmC promoter to inhibit expression of yvmC-cypX. Our data suggests ScoC can bind to the promoter but does so weakly. Thus, it is possible its mode of regulation may be direct or indirect through an unidentified transcriptional regulator controlled by ScoC. In any case, as nutrients become limiting, the transition state regulators become inactive, relieving repression on the yvmC promoter. As a MarR family transcription factor, PchR activity is likely regulated by a small ligand of which the identity is currently not known.