Homologous Introns Research Articles

Nucleotide sequence and intron structure of the apocytochrome b gene of Neurospora crassa mitochondria.

The sequence of the apocytochrome b (cob) gene of Neurospora crassa has been determined. The structural gene is interrupted by two intervening sequences of approximately 1260 bp each. The polypeptide encoded by the exons shows extensive homology with the cob proteins of Aspergillus nidulans and Saccharomyces cerevisiae (79% and 60%, respectively). The two introns are, however, located at sites different from those of introns in the cob genes of A. nidulans and S. cerevisiae (which contain highly homologous introns at the same site within the gene). The introns share several short regions of sequence homology (10-12 bp long) with each other and with other fungal mitochondrial introns. Moreover, the second intron contains a 50 nucleotide long sequence that is highly homologous with sequences within every ribosomal intron of fungal mitochondria sequenced to date. The conserved sequences may allow the formation of a core secondary structure, which is nearly identical in many mitochondrial introns. The conserved secondary structure may be required for intron splicing. The second intron contains an open reading frame, continuous with the preceding exon, of approximately 290 codons. Two stretches of 10 amino acid residues, conserved in many introns, are present in the open reading frame.

Single base substitution in an intron of oxidase gene compensates splicing defects of the cytochrome b gene

An extragenic suppressor mutation, mim2-1, which compensates yeast mitochondrial mutants deficient in splicing of the cytochrome b gene, has been mapped and sequenced. The mutation is due to a single G leads to A transition in the long open reading frame of the fourth intron of the oxidase subunit one gene. It causes the replacement of a glutamic codon by a lysine codon and the expression of a novel mRNA maturase active in splicing. Evolution and regulatory connections between homologous introns of nonhomologous genes are discussed.

Comparison of cloned rabbit and mouse β-globin genes showing strong evolutionary divergence of two homologous pairs of introns

Cloned beta-globin genes of both mouse and rabbit each contain a large and a small intervening sequence (intron) of about equal length at precisely the same positions relative to the coding sequence. The homologous introns show some sequence similarity, particularly at the junctions with the coding sequence. They most probably arose from a common ancestral sequence and diverged substantially during evolution.