A histidine gene cluster of the hyperthermophile Thermotoga maritima: sequence analysis and evolutionary significance

The sequences of histidine operon genes in hyperthermophiles are informative for understanding high protein thermostability and the evolution of metabolic pathways. Therefore, a cluster of eight his genes from the hyperthermophilic and phylogenetically early bacterium Thermotoga maritima was cloned and sequenced. The cluster has the gene order hisDCBdHAFI-E, lacking only hisG and hisBp, and does not contain intercistronic regions. This compact organization of his genes resembles the his operon of enterobacteria. Sequence analysis downstream of the stop codon of hisI-E identifies a region with a significantly higher cytosine over guanosine content, which is indicative of a rho-dependent termination of transcription of the his operon. Multiple sequence alignments of N1-((5'-phosphoribosyl)-formimino)-5-aminoimidazole-4-carboxyam ide ribonucleotide isomerase (HisA) and of the cycloligase moiety of imidazoleglycerol phosphate synthase (HisF) support the previous assignment of the (beta alpha)8-barrel fold to these proteins. The alignments also reveal a second phosphate-binding motif located in the first halves of both enzymes and thereby support the hypothesis that HisA and HisF have evolved by a sequence of two gene duplication events. Comparison of the amino acid compositions of HisA and HisF from mesophiles and thermophiles shows that the thermostable variants of both enzymes contain a significantly increased number of charged amino acid residues and may therefore be stabilized by additional salt bridges.