Putative mechanism of natural transformation as deduced from genome data

Genetic transformation is widely utilized in molecular biology as a tool for gene cloning in Escherichia coli and for gene mapping in Bacillus subtilis. Several strains of eubacteria can naturally take up exogenous DNA and integrate the DNA into their own genomes. Molecular details of natural transformation, however, remained to be elucidated. The complete genome of a cyanobacterium, Synechocystis sp. PCC6803, has been sequenced. This bacterium has been used to examine functions of a particular gene. The genome is considered to carry information on natural transformable characteristics of Synechocystis. The first step in genetic transformation is the uptake of exogenous DNA. Proteins with non-specific DNA binding features are required, because specificity in the exogenous DNA has not been demonstrated. Such proteins have modules interacting with the phosphate backbone of DNA, including helix-turn-helix modules. Using a consensus pattern of the phosphate-binding helix-turn-helix module, we searched through the genome data of Synechocystis for genes or open reading frame (ORF) products with the pattern in primary structures. We found that an ORF, slr0197, has the pattern in duplicate at the C-terminal region. We also found that the ORF product has a hydrophobic segment at the N-terminal region, which is followed by two internal repeats of the endonuclease domain. Based on these observations, we propose a model for the initial stage of genetic transformation. This is apparently the first report on molecular mechanisms of natural transformation.