In vivo interactions of the NahR transcriptional activator with its target sequences. Inducer-mediated changes resulting in transcription activation.

The nahR gene from the NAH7 naphthalene degradation plasmid encodes a LysR-type transcriptional activator of the nah and sal promoters (Pnah and Psal, respectively) that responds to the inducer salicylate. In vivo methylation protection experiments with dimethyl sulfate showed that in the absence of inducer, NahR interacts in a similar manner with its target sites at Psal and Pnah. Both target sites also have very similar sequences comprised of a 4-base pair interrupted dyad containing two symmetrical guanines (-73 and -64 of Pnah; -71 and -62 of Psal), each located in adjacent major grooves on the same helical face, and both strongly protected by NahR. When inducer was present, several additional guanines of Pnah (-35, -45, and -58) and Psal (-42 and -40) became protected from methylation, while a guanine at -52 of Pnah became markedly enhanced for methylation, indicating that inducer and NahR-dependent interactions with these downstream sites of each promoter are quite different. Deletion of Psal sequences downstream of -30 did not affect its methylation patterns suggesting that NahR alone is responsible for the altered reactivities of these nucleotides. Similar in vivo methylation analyses with inducer-insensitive or inducer-independent NahR mutants also suggested that all alterations in methylation sensitivity are directly caused by NahR. It is more probable that the salicylate-induced reactivity changes result from direct NahR-guanine contacts which are required for, but not sufficient for transcription activation; however, they could also result from NahR-induced DNA contortions caused by upstream protein-DNA contacts.