What components of the introns of nuclear genes that encode proteins in higher eukaryotes are conserved and required for the correct excision of intro sequences from primary transcripts by spliceosomes?

The introns of protein-encoding nuclear genes of higher eukaryotes almost invariably begin (5') with GT and end (3') with AG. In addition, the 3'subterminal A in the “TACTAAC box” is completely conserved; this A is involved in bond formation during intron excision.

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