Intein harbouring large tandem repeats in replicative DNA helicase of Trichodesmium erythraeum

Protein splicing inteins can be small as approximately 130 aa or up to approximately 600 aa when harbouring an endonuclease domain. Here we report the identification and characterization of an unusually large intein, 1650 aa long and the largest of known inteins, encoded by the replicative DNA helicase gene dnaB of the oceanic N2-fixing cyanobacterium Trichodesmium erythraeum. This Ter DnaB-1 intein co-exists with a 177-aa mini-intein in the same host protein and harbours large tandem repeats in which an 84-aa sequence is repeated 16 times. Comparison between this tandem repeats and the recently reported tandem repeats of Ter DnaE-1 intein revealed differences and similarities. The two tandem repeats, residing in different inteins of different host proteins, differ by 50% in size and have little sequence similarity. Tandem repeats in the Ter DnaB-1 intein were required for the protein splicing activity when tested in Escherichia coli, in contrast to tandem repeats of the Ter DnaE-1 intein that inhibited protein splicing. On the other hand, tandem repeats of both inteins are located in the same corresponding region of the intein sequence and have the same number of repeating units. These suggest that the two tandem repeats could be related but have diverged greatly in size, sequence and effect on protein splicing. Alternatively, they could have independent origins but evolved certain similarities because of common constraints in structure and maintenance.