Fig. 4.

Models of NolR regulation of nodulation and symbiosis gene expression. (A) In promoters with overlapping transcription initiation and operator sites, NolR binding prevents RNA polymerase interaction and gene expression. (B) In promoter regions containing upstream nod box sequences for binding of the transcriptional activator NodD, binding of NolR to the operator site may either alter association of NodD to the nod box or alter DNA bending that results from NodD binding to prevent activation of gene expression.