Fig. 3. Intron positions of pyruvate kinase genes in relation to structural features of the 530-residue feline muscle enzyme (28). (A) Centrality plot (average centrality =14.6 Å). PK has four domains (B), with boundaries defined using secondary structural elements assigned to domains by Muirhead et al. (28). Domain A, an eight-stranded b-barrel (similar in structure and overall dimensions to TPI) is interrupted by domain B, and is flanked by separate amino and carboxy domains (N and C, respectively). (C) Elements of secondary structure. The 16 known intron positions (D) are represented in a minimal set of three genomic sequences (26), as follows: chicken: 51-1, 82-0, 126-0, 188-1, 278-2, 329-0, 380-0, 435-2, 496-1, Aspergillus: 51-1, 66-1, 82-0, 114-0, 156-2, 433-2, 461-0; potato: 115-0, 270-0 . Other conventions are as indicated for Fig. 1. Gilbert and Glynias (10) recently predicted that an intron would be found in a PK gene near codon 222, based on the nine positions from the chicken gene. However, none of the 7 additional positions known from Aspergillus and potato are closer than 48 codons to this site.