Dna

Intron

Promoter Exon 1

■ Coding sequence ■ within intron

Exon 2

FIGURE 4.04 Intron Containing a Coding Sequence of Its Own

An interesting situation can arise when a coding sequence has an intron within it that itself includes a coding sequence for another protein.

Proteins

Intron

■ Coding sequence ■ within intron

Exon 2

Protein coded by exon 1 plus exon 2

Protein coded by sequence inside intron

Protein coded by exon 1 plus exon 2

Protein coded by sequence inside intron genes have multiple introns. Furthermore, most known examples are within the genes of bacterial viruses, rather than the chromosomal genes of bacteria themselves. For example, bacteriophage T4 possesses several introns, including one each in the genes encoding thymidylate synthase and ribonucleotide reductase. The T4 introns are homologous to the self-splicing introns of lower eukaryotes (for splicing mechanisms, see Ch. 12). This family of introns takes the complexity one step further, as there is a coding sequence for a separate protein located entirely within the intron (Fig. 4.04). This protein is concerned with survival of the intron.

In those rare cases where chromosomal genes of prokaryotes are interrupted, the genes often encode RNA molecules rather than proteins. Both tRNA and rRNA genes have been found with introns in both the eubacteria and archaebacteria. For example, one of the leucine tRNA genes of cyanobacteria (blue-green photosynthetic bacteria) and the corresponding gene in the DNA of chloroplasts contain self-splicing introns inserted at equivalent positions.

Repeated sequences are frequently found in the DNA of higher organisms.

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