Organization of the chromosomal region containing the genes lexA and topA in Thermotoga neapolitana. Primary structure of LexA reveals phylogenetic relevance

The chromosomal region of Thermotoga neapolitana surrounding the gene lexA (4283 bp) was sequenced. In addition to the topoisomerase gene top2A it con tained five open reading frames. A part of the cloned region showed high se quence homology with a previously published sequence of Th. maritima and in dicated an identical arrangement of genes in both microorganisms. Structura l analysis of the LexA protein showed significant, but relatively low overa ll homology with LexA proteins of other bacteria, especially in the DNA bin ding region. However, key amino acids for processing and secondary structur e elements like the helix-turn-helix motif are well conserved. Sequence ali gnment analysis of the whole protein and the DNA-binding sites of all known LexA sequences uncovers groups of similarity reminding the phylogenetic tr ee of the Bacteria. A consensus sequence with the SOS- or Cheo-box upstream of the lexA gene of Th. maritima and Th. neapolitana was absent. Together with the phylogenetic distance of the Thermotogales from other bacteria thi s suggests the presence of a new operator target sequence specific for the Thermotogales, in analogy to the SOS-box for the gamma-group Proteobacteria and the Cheo-box for low- and high-GC Gram-positive bacteria.