Sequence Divergence of an Archaebacterial Gene Cloned from a Mesophilic and a Thermophilic Methanogen

摘要

A 1.6-kb fragment of DNA from the thermophilic, methane-producing, anaerobic archaebacterium Methanobacterium thermoautotrophicum DeltaH has been cloned and sequenced. The DNA complements mutations in both the purE1 and purE2 loci of Escherichia coli. The sequence of the M. thermoautotrophicum DNA predicts that complementation in E. coli results from the synthesis of a polypeptide with a molecular weight of 36,249. A polypeptide apparently of this molecular weight is synthesized in E. coli minicells containing recombinant plasmids that carry the cloned fragment of methanogen DNA. A purE-complementing gene from the mesophilic methanogen Methanobrevibacter smithii has been previously cloned and sequenced. The two methanogen-derived purE-complementing genes are 53% homologous and encode polypeptides that are 45% homologous in their amino acid sequences but would be 74% homologus if conservative amino acid substitutions were considered as maintaining sequence homology. The genome of M. thermoautotrophicum has a molar G + C content of 49.7%, whereas the genome of M. smithii is 30.6% G + C. Conservation of encoded amino acids while accommodating the very different G + C contents is accomplished by use of different codons that encode the same amino acid. (Copyright (c) Springer-Verlag 1985.)