Tryptophan gene cluster of Methanobacterium thermoautotrophicum Marburg: molecular cloning and nucleotide sequence of a putative trpEGCFBAD operon

Abstract

A recombinant cosmid carrying the Methanobacterium thermoautotrophicum Marburg trp genes was selected by complementation of Escherichia coli trp mutations. A 7.3-kb fragment of the cloned archaeal DNA was sequenced. It contained the seven trp genes, arranged adjacent to each other in the order trpEGCFBAD. No gene fusions were observed. The trp genes were organized in an operonlike structure, with four short (5- to 56-bp) intergenic regions and two overlapping genes. There was no indication for an open reading frame encoding a leader peptide in the upstream region of trpE. The gene order observed in the M. thermoautotrophicum trp operon was different from all known arrangements of the trp genes in archaea, bacteria, and eucarya. The encoded sequences of the Methanobacterium Trp proteins were similar in size to their bacterial and eucaryal counterparts, and all of them contained the segments of highly similar or invariant amino acid residues recognized in the Trp enzymes from bacteria and eucarya. The TrpE, TrpG, TrpC, TrpA, and TrpD proteins were 30 to 50% identical to those from representatives of other species. Significantly less sequence conservation (18 to 30%) was observed for TrpF, and TrpB exhibited a high degree of identity (50 to 62%) to the sequences of representatives of the three domains. With the exception of TrpB, the beta subunit of tryptophan synthase, tryptophan was absent from all Trp polypeptides.