Sulfur homeostasis in diverse microbes is governed by FinR, a LysR-type transcriptional regulator

Prokaryotic regulation of cysteine biosynthesis has long been associated with LysR-type transcriptional regulators (LTTRs) with designations such as CysB, Cbl, and CysR. However, the diversity of proteins in this regulatory family, as well as inconsistencies in nomenclature, complicate interpretation of the functional relationship among LTTRs in different bacteria. In this work, we report crystal structures of the full-length regulator FinR from Acinetobacter baylyi ADP1 and an effector-binding domain of YeiE from Escherichia coli. These proteins share a sulfite-recognition motif that is widely distributed among bacterial phyla and a few Archaea. Sulfite-dependent transcriptional activation was demonstrated in vitro at the cysI , cysDN , and fpr2 promoters of A. baylyi and fprA promoter of P. aeruginosa . Our results reveal that these proteins represent a distinct class of regulators involved in sulfur metabolism in prokaryotes that may encompass roles in cysteine synthesis, detoxification, and redox control.