Nodule inception initially relies on cell competence in a narrow infection zone located just behind the growing root tip. Older nodules then regulate the number of nodules on a root system by suppressing the development of nodule primordia.Nin (for nodule inception) is required for the formation of infection threads and the initiation of primordia. NIN protein has regional similarity to transcription factors, and the predicted DNA-binding/dimerization domain identifies and typifies a consensus motif conserved in plant proteins with a function in nitrogen-controlled development.
The full-length Nin cDNA has an open reading frame of 2,634 bp, a 5' leader with stop codons in all three reading frames, and a 3' untranslated region of 141 bp. The protein, comprising 878 amino acids, is shown in Fig. 3a along with a diagram of several domains with similarity to transcription factors. The carboxy-terminal half of the protein carries an unusual putative DNA-binding domain with a basic region followed by two heptad leucine or isoleucine repeats (R1 and R2); an amphiphatic leucine-zipper structure was predicted for R2 by helical-wheel analysis (Fig. 3). This arrangement is similar to the DNA-binding/dimerization domains of the bZIP8 and bHLH/Z9 transcription factors, but the R1/R2 region does not comply with a strict consensus of either domain. The distance between the basic region and the zipper, for example, is longer than would be expected from the bZIP consensus. The presence of a proline helix breaker (Pro 597) in R1 also distinguishes NIN, and alignment to prokaryotic helix