A PCR assay for the amplification of small subunit ribosomal DNA (SSU rDNA) of Euryarchaea was developed and used to detect archaeal rDNA in 37 (77%) out of 48 pooled subgingival plaque samples from 48 patients suffering from periodontal disease. One major group of cloned periodontal sequences was identical to Methanobrevibacter oralis and a second minor group to Methanobrevibacter smithii. These two groups and a third novel group were found to be more than 98% similar to each other over an 0.65-kb segment of the 16S rRNA gene sequenced. M. oralis was found to be the predominant archaeon in the subgingival dental plaque. Phylogenetic analysis of partial SSU rDNA sequences revealed evidence for a distinct cluster for human and animal Methanobrevibacter sp. within the Methanobacteriaceae family. ß

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The bacteriophage P1 genome carries an invertible C segment consisting of 3‐kb unique sequences flanked by 0.6‐kb inverted repeats. With insertion and deletion mutants of P1 derivatives the site‐specific recombinase gene cin for C inversion) has been mapped adjacent to the C segment and the cix sites (for C inversion cross‐over) have been located at the outside ends of the inverted repeats. Inversion of the C segment functions as a biological switch and controls expression of the gene(s) responsible for phage infectivity carried on the C segment. The cin gene product can promote recombination between a ‘quasi‐ cix’ site on plasmid pBR322 and a cix site on P1 DNA. The junctions formed on the resulting co‐integrate can also serve as cix sites. This observation implies a potential evolutionary process to bring genes under the control of a biological switch acting by DNA inversion.

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The kanamycin resistance transposon Tn2680, which originates from the R plasmid Rtsl, is homologous to Tn6 and carries two directly repeated copies of IS26, one at each end. The kanamycin resistance gene codes for type I aminoglycoside-3'-phosphotransferase. Tn2680 also contains, in the middle of the transposon, an additonal IS element homologous to IS903. This element, designated IS903.B, is flanked by a 9-base-pair direct target duplication. A novel kanamycin resistance transposon, Tn2681, can be generated from Tn2680 by IS903.B-mediated cointegration and subsequent reciprocal recombination between the directly repeated IS26 sequences. Tn2681 carries a single IS26 element in the middle of the transposon and is flanked by two directly repeated copies of IS903.B. Possible evolutionary relationships between Tn2680 and other kanamycin resistance transposons such as Tn903 and Tn2350 are discussed, based on the gene organization and DNA sequences.