Rna

Exon 1 Intron Exon 2 Extein 1 Intein Extein 2

Splicing of rna mRNA Exon 1 Exon 2

Translation Translation

Protein

From

From

From extein 2

Splicing of protein

From extein 2

Splicing of protein

FIGURE 12.17 Inteins and Exteins in Proteins

On the left is the standard scheme by which introns are eliminated during RNA splicing. The intron is eliminated at the level of RNA and is never translated into protein. On the right is the scheme for removal of intervening sequences at the protein level. Regions remaining in the final protein are called exteins and those destined to be lost are called inteins. The major difference from RNA splicing is that in protein splicing the inteins are cut out after the protein is made.

dnaE gene of Synechocystis (a blue-green bacterium).This gene is split in two and each half has part of the DNA sequence for an intein attached. These two half-genes are transcribed and translated separately to give two proteins (Fig. 12.19). These fold up together and even though the intein is separated into two segments it still manages to cut itself out. The two halves of the DnaE protein are joined together as the intein splices itself out. The intein is released as two fragments.

If the DNA segment that codes for the intein were deleted, the protein would be made in one step, with no intein and no need for protein splicing. And the intein itself would be extinct. However, the spliced-out intein polypeptide is not just a waste product; it is a site-specific deoxyribonuclease (DNase). Its function is to protect the

Protein prior to splicing

N-terminus

N-terminus

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