Mycoplasma pneumoniae, a bacterium pathogenic for humans, has a relatively small genome size of 840 kbp. Even though, several repeated DNA elements have been identified in the genome of this prokaryote, particularly within the P1 gene which codes for a major adhesin protein of M. pneumoniae. These elements were characterized in detail with respect to size, number and distribution on the genome, represented by an ordered cloned library covering the complete chromosome. Three different repetitive elements were detected in and around the P1 gene designated as RepMP2/3, RepMP4 and RepMP5. The length of these elements varies between 1.1-1.5 kbp (RepMP4), 1.8 kbp (RepMP2/3) and 1.9-2.2 kpb (RepMP5). They occur at least 8 to 10 times on the chromosome. Possible functions are discussed and a uniform nomenclature for these repeats is proposed.

The OR1F6 gene product of Mycopklsma pneumonae is involved in a yet-unknown manner in the adhesion of the bacterium to its host cell. Part of the ORF6 gene is a repetitive DNA sequence (RepMP5), about 1,900 bp Iong. Seven additiohal similar copies of RcpMPS are dispersed on the genome. In the independently isolated strains M. pneumoniae M129 and FH, the RepMP5 .opies residing in the ORF6 gene are not identical. Two conserved regions, ranging from nucleotides 1 to 799 and from nucleotide 1795 to the end-of the gene, border a vairiable region, ran,lng from nucleotides 800 to 1794. This variable region. differs in DNA sequence and by 201 bp. Analysis,of R'pMP5 copies outside the ORF6 geine shewed that both M. pneumoniae M129 and M.pneumoniae FH carry a RepMP5 copy on a 6-kbp EcoRI fragment which has the same DNA sequence as the yariable region of RepMP5 in the M. pneumoniae FH ORF6 gene. According to these data, a switch from the M. pneumoniae M129 bRF6 gene to the M. pneumoniae FH ORF6 gene could take place by gene conversion.The bacterium Mycoplasma pneumoniae is the causative agent of a primary atypical pneumonia in humans (16,36). As a surface parasite, it has to ensure a firm interaction with its host cell and to find ways to evade the immune response of the host. The mecharlisms M. pneumoniae and other mycoplasma species use to cope with the immune system of the host are still not understood, but it is well documented that M. pneumoniae persists in humans, even after successful chemothhrApy (15).The prerequisite for persistence is the ability of these bacteria to adhere to their host cells (22). Adherence of M. pneumoniae is mediated by adhesins which bind to the host receptor. They are still ill-defined. There is more than one type of receptor.Sialoglycoproteins (19,20,23), sulfated glycolipids.(17), or a sialic acid-free glycoprotein (10) have been identified as receptors or as components of receptors.The major bacterial adhesin is the 170-kDa protein P1 (11,12). In addition, a 30-kDa protein has been sdggested to be involved in adherence (5). Both proteins are part of a tip-like organelle (2, 7). If this structure is not properly asserhbled-if, for instance, the P1 protein is missing-then the bacterium is avirulent. The P1 protein is coded for by the P1 gene (13, 35), which is part of the Pl operon (14). According to DNA sequencing data, the P1 operon consists of three open reading frames, ORF4 and ORF6 flanking the Pl gene (ORF5 are located in the P1 gene (RepMP2/3 and RepMP4) (26,32,38), and a third one (RepMP5) is located in the ORF6 gene (see Fig. 1) (4, 26). Su et al. (33,34) classified the species M. pneumoniae into two groups (I and II) based upon different hybridization patterns of the Pl gene carrying DNA fragments in Southern blots of restricted genomic DNA of independently isolated strains. The prototype of group I is M. pn"eutnoniae M129, and that of group II is M. pneumoniae FH.DNA sequence comparison between the P1 gene of M.pneumoniae FH and that of M. pneumoniae M129 showed differences at t...

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