Intron/exon structure of the chicken pyruvate kinase gene.
The chicken pyruvate kinase gene is interrupted by at least ten introns, including nine introns within the coding region. We compare the structure of this gene with the three-dimensional protein structure of the homologous cat muscle enzyme. The introns are not randomly placed--they divide the coding sequence into fairly uniformly sized pieces encoding discrete elements of secondary structure. The introns tend to fall at interruptions between stretches of alpha-helix or beta-sheet residues, and each of the six exons that contribute to the barrel-shaped central domain include one or two repeats of a simple unit, an alpha-helix plus a beta strand. This structure suggests that introns were not inserted into a previously uninterrupted coding sequence, but instead are products of the evolution of the first pyruvate kinase gene. We have found some sequence homology between a segment of pyruvate kinase and the structurally homologous mononucleotide binding fold of alcohol dehydrogenase. The superposition of these two regions aligns an intron from the maize alcohol dehydrogenase gene four nucleotides from an intron in the chicken pyruvate kinase gene.