The nucleotide sequence of a nodule-specific gene, Nms-25 of Medicago sativa: its primary evolution via exon-shuffling and retrotransposon-mediated DNA rearrangements.

We present the primary structure of a nodule-specific gene, Nms-25 from Medicago sativa L. cultivar Nagyszénási. Analysis of the nucleotide sequence of Nms-25 revealed that this gene shows all the characteristics of an interrupted plant gene consisting of 13 exons and 12 introns. The promoter region of Nms-25 contains the common promoter elements of plant genes as well as motifs which are supposed to be involved in nodule-specific expression. There are two exon-like sequences in the gene named PE1 and PE2 which are not present in the cDNA clones of Medicago sativa cultivar Cardinal. Intron 9 carries a retrotransposon-like element, Tms1, which might be responsible for downstream deletion events in which a heptanucleotide, ATTAGCT, might have been involved. Most of the exons, except 1, 12 and 13, are similar to each other both in length (54 bp) and sequence (up to 94% sequence similarity). All exons are interrupted by introns in the same phase (type I). It is suggested that exon-shuffling based on illegitimate recombination in which the ATTAGCT motif might have played an active role, and retrotransposon-mediated DNA rearrangements were the primary events in the molecular evolution of the Nms-25 gene.