Representatives of the family were analyzed phylogenetically by comparing partial sequences of their methyl-coenzyme M reductase (I) genes. A 490-bp fragment from the A subunit of the gene was selected, amplified by the PCR, cloned, and sequenced for each of 25 strains belonging to the The sequences obtained were aligned with the corresponding portions of five previously published sequences, and all of the sequences were compared to determine phylogenetic distances by Fitch distance matrix methods. We prepared analogous trees based on 16S rRNA sequences; these trees corresponded closely to the I trees, although the I sequences of pairs of organisms had 3.01 ± 0.541 times more changes than the respective pairs of 16S rRNA sequences, suggesting that the I fragment evolved about three times more rapidly than the 16S rRNA gene. The qualitative similarity of the I and 16S rRNA trees suggests that transfer of genetic information between dissimilar organisms has not significantly affected these sequences, although we found inconsistencies between some I distances that we measured and previously published DNA reassociation data. It is unlikely that multiple I isogenes were present in the organisms that we examined, because we found no major discrepancies in multiple determinations of I sequences from the same organism. Our primers for the PCR also match analogous sites in the previously published II sequences, but all of the sequences that we obtained from members of the were more closely related to I sequences than to II sequences, suggesting that members of the do not have distinct II genes.


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