Evolution of legumin genes: loss of an ancestral intron at the beginning of angiosperm diversification

The polymerase chain reaction was used to survey gymnosperm legumin genes. Characterization of 46 cloned amplificates, differing in sequence and size (1.2-1.6 kb), revealed the ubiquitous occurrence of legumin genes and their organization in small subfamilies in the 22 species investigated. The 3' portions of the genes, coding for the legumin beta-polypeptides, show a highly conserved intron/exon structure divergent from those of angiosperms: an additional intron (intron IV) uniformly interrupts the region coding for the C-terminal part of the beta-polypeptides. Phylogenetic analysis of the respective coding sequences as well as the organization of the Magnolia B14 legumin gene also investigated here both indicate that intron IV is ancestral and was lost during early angiosperm evolution. Taking into account the intron/exon structures from all legumin genes known, our results suggest that legumin genes evolved by subsequent loss of introns, providing also further evidence for a common origin of legumins and vicilins.