PARTIAL GENE-SEQUENCES FOR THE A-SUBUNIT OF METHYL-COENZYME-M REDUCTASE (MCRI) AS A PHYLOGENETIC TOOL FOR THE FAMILY METHANOSARCINACEAE

Representatives of the family Methanosarcinaceae were analyzed phyloge netically by comparing partial sequences of their methyl-coenzyme M re ductase (mcrI) 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 Methanosarcinaceae. The sequences obtaine d were aligned with the corresponding portions of five previously publ ished sequences, and all of the sequences were compared to determine p hylogenetic distances by Fitch distance matrix methods, We prepared an alogous trees based on 16S rRNA sequences; these trees corresponded cl osely to the mcrI trees, although the mcrI sequences of pairs of organ isms had 3.01 +/- 0.541 times more changes than the respective pairs o f 16S rRNA sequences, suggesting that the mcrI fragment evolved about three times more rapidly than the 16S rRNA gene, The qualitative simil arity of the mcrI and 16S rRNA trees suggests that transfer of genetic information between dissimilar organisms has not significantly affect ed these sequences, although we found inconsistencies between some mcr I distances that we measured and previously published DNA reassociatio n data. It is unlikely that multiple mcrI isogenes were present in the organisms that we examined, because we found no major discrepancies i n multiple determinations of mcrI sequences from the same organism, Ou r primers for the PCR also match analogous sites in the previously pub lished mcrII sequences, but all of the sequences that we obtained from members of the Methanosarcinaceae were more closely related to mcrI s equences than to mcrII sequences, suggesting that members of tile Meth anosarcinaceae do not have distinct mcrII genes.