The nodA promoter of the symbiotic plasmid pRL1JI. The transcription start sites for the nodA and nodD genes are indicated by arrows (8,9). The nodA transcription start site is numbered +1. The consensus sequence of the canonical nod box is boxed (9,14). (A) The minimal inducible nodA promoter. The conserved sequence ATAG, which is part of the nodA promoter, is in bold (9). Lines indicate the DNase I footprints of NodD, and triangular arrowheads denote hypersensitive sites (8). The two protected half-sites of the nod box separated by the hypersensitive sites are named D-half and P-half, respectively. (B) The nodA promoter deletions cloned between the BamHI–EcoRI sites of pBluescriptKSII-derived plasmids pBSb, pBSc, pBSd, pBSe and pBSf (see Materials and Methods). The 5′ ends of the deletions are different and marked by the vertical arrows, and the 3′ ends are all at position +57. The upper case letters represent the nodA promoter region, and the lower case letters are related to relevant plasmids. The BamHI, PstI and EcoRI sites are in bold. (C) The nodA promoter (–75 to +57) with an O13 mutant D-half cloned in the PstI–EcoRI sites of pUO13 (see Materials and Methods). The four mutated residues in the O13 D-half are in bold. The PstI and EcoRI sites are also in bold.