Relationship between the total size of exons and introns in protein-coding genes of higher eukaryotes.

Abstract

The correlation between the genetic information content in the exons and the surrounding intron sequences was investigated with regard to their spatial arrangement within a gene. A comparison is made of the sizes, taken from recent publications, of exons and introns of .apprxeq. 80 different protein-coding chromosomal genes, mostly from higher eukaryotes. The exons of these genes do not show very marked variation in size and can be classified into 3 major discrete and 2 minor additional size groups, whereas individual introns vary considerably in size within and between genes. Notwithstanding, the overall length of all introns present within a given gene is a function of the total size, mostly corresponding to the total genetic information content, of the exons. Three cases that violate this exon-size dependency of introns are genes coding for histone H1, feather keratin and interferons; tubulin and actin; and silk fibroin. The exons of these genes are larger than 0.7 kilobase pair in total size and the genes show a strong sequence homogeneity among the repetitious family members or internal repeats of coding sequences within the gene. Conservation of sequences, which is required by the family members, internal repeats, or the entire gene, might actually motivate the removal of introns.