LexA-binding sequences in Gram-positive and cyanobacteria are closely related

The lexA gene of the cyanobacterium Anabaena sp. strain PCC7120 has been cloned by PCR amplification with primers designed after TBLASTN analysis of its genome sequence using the Escherichia coli LexA sequence as a probe. After over-expression in E. coli and subsequent purification, footprinting experiments demonstrated that the Anabaena LexA protein binds to the sequence TAGTACTAATGTTCTA, which is found upstream of its own coding gene. Directed mutagenesis and sequence comparison of promoters of other Anabaena genes, as well as those of several cyanobacteria, allowed us to define the motif RGTACNNNDGTWCB as the LexA box in this bacterial phylum. Substitution of a single nucleotide in this motif present in the Anabena lexA promoter is sufficient to enable it to bind the Bacillus subtilis LexA protein. These data indicate that Cyanobacteria and Gram-positive bacteria are phylogenetically closely related.