The RNA polymerase of Methanococcus vannielii, in binary complex with two stable RNA operons, protects from exonuclease digestion the region from 32 bp upstream (-32) to 18 bp downstream (+18) of the transcription start site. Contained within this binding region, centered at -25, is an AT-rich sequence which is highly conserved upstream of 26 other archaebacterial tRNA and rRNA genes. We therefore propose the sequence TTTATAATA as a common element of promoters for stable RNA genes in archaebacteria. Both the similarity in sequence and the location of this conserved octanucleotide suggest homology to the eukaryotic TATA box preceding protein encoding genes transcribed by RNA polymerase B.