Conserved gene structures and expression signals in methanogenic archaebacteria.

A comparative analysis of cotranscribed gene clusters comprising the structural genes mcrA, mcrB, mcrC, mcrD, and mcrG was carried out in three species of methanogens. mcrA, mcrB, and mcrG are the structural genes for the three subunits of methyl coenzyme M reductase, while the two other genes encode polypeptides of unknown functions. The degree of conservation of the mcr gene products among different species of methanogens varies. No correlation was found between the conservation of the G+C contents of the homologous genes and of the amino acid sequences of their products among the different bacteria. The comparison of RNA polymerase core subunit genes of Methanobacterium thermoautotrophicum as evolutionary markers with their equivalents in Escherichia coli, Saccharomyces cerevisiae, and Drosophila melanogaster showed that homologous polypeptide domains are encoded by different numbers of genes suggesting gene fusion of adjacent genes in the course of evolution. The archaebacterial subunits exhibit much stronger homology with their eukaryotic than with their eubacterial equivalents on the polypeptide sequence level. All the analyzed genes are preceded by ribosome binding sites of eubacterial type. In addition to known putative promoter sequences, conserved structural elements of the DNA were detected surrounding the transcription initiation sites of the mcr genes.