CHARACTERIZATION OF A NOVEL NODULIN GENE IN SOYBEAN THAT SHARES SEQUENCE SIMILARITY TO THE GENE FOR NODULIN-24

A gene encoding for nodulin-16 (N-16) was isolated from a soybean genomic library. Nucleotide sequence analysis of the cDNA and the genomic clone of N-16 indicated that the coding region of this gene is 330 bp long and is interrupted by a single intron of 494 bp. The coding region of the N-16 gene shows a high degree of localized sequence similarity with the coding sequence of soybean nodulin-24 (N-24). Sequence similarity between the two genes is limited to the coding region of 90 bp in the first exon and the first 54 bp in the second exon of the N-16 gene which is repeated as the 2nd, 3rd, and 4th exons in the N-24 gene. The N-24 gene has been postulated to be a result of repeated duplication of an insertion element consisting of the 54 bp exon and the flanking intron sequences. In the absence of sequence similarity in the regions flanking the 54 bp sequence between the N-16 and N-24 genes, the N-16 gene does not appear to be ancestral gene. Both N-16 and N-24 have a similar hydrophobic amino terminal end suggesting that N-16 like N-24 is targeted to the peribacteroid membrane. Southern analysis of soybean genomic DNA shows the presence of other related sequences to the N-16 gene, one of which is found to be closely linked to it. Analysis of the temporal accumulation of the N-16 transcripts during nodule development in effective and ineffective nodules suggests that N-16 and related genes might differ from leghemoglobin and some other late nodulin genes in their mechanism of regulation.