A HISTIDINE GENE-CLUSTER OF THE HYPERTHERMOPHILE THERMOTOGA-MARITIMA - SEQUENCE-ANALYSIS AND EVOLUTIONARY SIGNIFICANCE

The sequences of histidine operon genes in hyperthermophiles are infor mative for understanding high protein thermostability and the evolutio n 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 hisD CBdHAFI-E, lacking only hisG and hisBp, and does not contain intercist ronic regions. This compact organization of his genes resembles the hi s operon of enterobacteria. Sequence analysis downstream of the stop c odon of hisI-E identifies a region with a significantly higher cytosin e over guanosine content, which is indicative of a rho-dependent termi nation of transcription of the his operon. Multiple sequence alignment s of N-1-((5'- phosphoribosyl)-formimino)-5-aminoimidazole 4-carboxyam ide ribonucleotide isomerase (HisA) and of the cycloligase moiety of i midazoleglycerol phosphate synthase (HisF) support the previous assign ment of the (beta alpha)(8)-barrel fold to these proteins. The alignme nts also reveal a second phosphate-binding motif located in the first halves of both enzymes and thereby support the hypothesis that HisA an d HisF have evolved by a sequence of two gene duplication events. Comp arison 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 residu es and may therefore be stabilized by additional salt bridges.