A cosmid bank of Methanococcus voltae DNA was obtained in Escherichia coli after ligation of partially HindIII-digested M. voltae DNA in the HindIII site of the transferable cosmid pVK100. The bank was used to perform complementation experiments with E. coli auxotrophic mutants. Five cosmids complementing trpA shared three adjacent HindIII fragments of 2.1, 2.3 and 14 kb. Two of these cosmids also complemented trpD and carried an additional 4.2 kb HindIII fragment. The trpA- and trpD- complementing regions were more precisely localized using Tn5 mutagenesis. A 1.7 kb PstI fragment, cloned into pUC9 in both orientations, was responsible for the trpA complementation. This fragment was sequenced and an open reading frame (ORF) of 852 nucleotides (ORFtrpA) encoding a 284 amino acid polypeptide of mol. wt. 31,938 was found. The amino acid sequence was compared with that of the alpha subunit of tryptophan synthase (trpA gene product) from nine eubacterial species and to the N-terminal part of the tryptophan synthase of Saccharomyces cerevisiae (TRP5 gene product). Similarity varied from 24% (Brevibacterium lactofermentum) to 35% (S. cerevisiae). The nucleotide sequence of the region upstream from M. voltae ORFtrpA was determined and revealed the presence of an ORF of 1227 nucleotides (ORFtrpB) encoding a 409 amino acid polypeptide of mol. wt. 44,634. The polypeptide sequence was similar to the beta subunit of tryptophan synthase (trpB gene product) from six eubacterial species and to the C-terminal part of the tryptophan synthase of S. cerevisiae. Similarity varied from 49% (S. cerevisiae, B. lactofermentum) to 58% (Pseudomonas aeruginosa). This high conservation supports the hypothesis of a common ancestor for the trpA and trpB genes of archaebacteria, eubacteria and eucaryotes. M. voltae ORFtrpA and ORFtrpB, which are transcribed in the same direction, are separated by a 37 bp AT-rich region. Immediately upstream from ORFtrpB, the 3' end of an ORF homologous to E. coli and Bacillus subtilis trpF was found. As the trpD-complementing region was located upstream from the trpFBA sequenced region, the organization of trp genes in the archaebacterium might thus be trpDFBA. Such an organization resembles that of enteric eubacteria, in which the trpEDCFBA genes are grouped in a single operon. However, M. voltae ORFtrpA and ORFtrpB do not overlap, in contrast with what is found in most eubacteria.