TI - Conclusions . AB - This study investigated the effects of sequence changes in the Fnr-recognition site on the anaerobic activation of dmsA-lacZ expression as well as examined the NarL recognition sites within the dmsABC regulatory region . The data illustrates that Fnr is responsible for the 100-fold anaerobic activation of dmsA expression . Also , both half sites of the Fnr recognition sequence at dmsA are required for Fnr -dependent expression and are similar in their ability to activate dmsA transcription . Furthermore , the spacing between the Fnr and RNA polymerase recognition sequences is critical at dmsA . In vitro interactions of the nitrate-responsive regulatory protein NarL with the promoter region of dmsABC were examined using DNase I and hydroxyl radical footprinting techniques . The location of the NarL-phosphate protected regions within a 97 bp segment of the dmsA promoter is consistent with the model for dmsABC expression whereby multiple molecules of NarL-phosphate recognize and bind the DNA in a weak and cooperative fashion . The NarL interactions with the dmsA promoter region occurred at ten bp intervals and were offset by 3 bp in the 3' direction , suggesting the assembly of multiple NarL-phosphate molecules onto one face of the DNA that protect the minor groove . Furthermore , nonphosphorylaTED NarL was unable to protect the NarL binding sequences at the dmsA promoter region , suggesting that PHOSphorylation of NarL is required for repression of dmsABC expression .