Restriction of an intron size <i>en route</i> to endothermy

Abstract Ca2+-insensitive and -sensitive E1 subunits of the 2-oxoglutarate dehydrogenase complex (OGDHC) regulate tissue-specific NADH ATP supply by mutually exclusive OGDH exons 4a 4b. Here we show that their splicing is enforced distant lariat branch points (dBPs) located near 5′ splice site intervening intron. dBPs restrict intron length prevent transposon insertions, which can introduce or eliminate dBP competitors. The size restriction was imposed a single dominant in anamniotes expanded into conserved constellation four adenines amniotes. amniote clusters exhibit taxon-specific usage individual dBPs, reflecting accessibility extended motifs within stable RNA hairpin rather than U2 snRNA:dBP base-pairing. expansion took place early terrestrial species followed uridine enrichment large downstream polypyrimidine tracts mammals. dBP-protected megatracts permit reciprocal regulation exon 4b uridine-binding proteins, including TIA-1/TIAR PUF60, promote U1 snRNP recruitment to BP, respectively, but do not significantly alter relative usage. We further codons for residues critically contributing protein binding sites Ca2+ other divalent metals confer inclusion order mirrors Irving-Williams affinity series, linking evolution auxiliary metallome constraints. Finally, hypothesize dBP-driven selection Ca2+-dependent provision facilitated endothermy optimizing aerobic scope target tissues.