To identify an archaebacterial promoter sequence, nuclease protection studies with the purified RNA polymerase ofMethanococcus vannieliiwere performed. The enzyme binds specifically both at protein-encoding (hisAand methyl CoM reductase, component C) and tRNA–rRNA genes. The binding region of the RNA polymerase extends from 30 base pairs (bp) upstream (−30) to 20 bp downstream (+20) from thein vivotranscription start site. This finding indicates that the archaebacterial enzyme recognizes promoters without transacting traascription factors. The DNA segment protected from nuclease digestion by bound RNA polymerase contains an octanucleotide sequence centered at −25, which is conserved between the protein-encoding and the stable RNA genes. According to the specific binding of the enzyme to only DNA-fragments harbouring this motif, we propose the sequence TTTATATA as the major recognition signal of theMethanococcusRNA polymerase. Comparison of this motif with published archaebacterial DNA sequences revealed the presence of homologous sequences at the same location upstream of 36 genes. We therefore consider the overall consensusas a general element of promoters in archaebacteria. In spite of the specific binding of the enzyme, most preparations of theMethanococcus vannieliiRNA polymerase are unable to initiate transcription at the correct sitesin vitro. Here we present first evidence for the possible existence of a transcription factor conferring the ability to the enzyme to initiate and terminate transcription specificallyin vitro.Key words: promoter, footprint, TATA box, RNA polymerase, transcription.