A DNAB INTEIN IN RHODOTHERMUS-MARINUS - INDICATION OF RECENT INTEIN HOMING ACROSS REMOTELY RELATED ORGANISMS

A dnaB gene encoding a homologue of the Escherichia coli DNA helicase DnaB was cloned and sequenced in the thermophilic eubacterium Rhodothe rmus marinus, predicting a DnaB protein that harbors an intein. This D naB intein is 428 amino acid residues long, has several putative intei n sequence motifs (including two putative endonuclease motifs), and is capable of protein splicing when produced in E. coli cells. The R. ma rinus DnaB intein is a close homologue of a DnaB intein in the cyanoba cterium Synechocystis sp. strain PCC6803. The two inteins are position ed identically in their respective DnaB proteins. They also share a 54 % sequence identity (74% sequence similarity) that is markedly higher than the 37% sequence identity shared by the extein sequences of the t wo DnaB proteins. Horizontal intein transfer (homing) is therefore inv oked to relate these two DnaB inteins. The codon usage of R. marinus D naB intein coding sequence differs markedly from the codon usages of i ts flanking extein coding sequences and other genes in the same genome , suggesting more recent acquisition of the DnaB intein in this organi sm.