MOLECULAR-BIOLOGY OF MYCOPLASMA

Mycoplasmas are the smallest free living microorganisms with the small est genome. The G+C content is in general low (25-33%) and the coding capacity is about 600 proteins. Mycoplasma species are phylogeneticall y related, they use the genetic codon UGA for tryptophan, and show rap id evolution, with a high rate of divergence. The genomes of Mycoplasm a genitalium and Mycoplasma pneumoniae have been fully sequenced. Stri king features of the M. genitalium sequencing project are the presence of a high number of membrane proteins with no resemblance to previous ly sequenced genes and the presence of repeated fragments of the gene encoding the tip-localized 140 kDa adhesin (MgPa). Many Myco- plasma s pecies display a high frequency of antigenic variation, both as phase and size variation of individual antigens. Mycoplasma hominis isolates are known to be antigenic heterogeneous, as reflected in the reactivi ty with monoclonal antibodies (MAbs). The genetics of the antigenic va riation has been studied for three different surface exposed antigens: P120, Lmp, and P50/Vaa, The gene encoding P120 had a hyper-variable r egion in the N-terminal region. In addition, a second gene with homolo gy to p120 was identified. The gene encoding Lmp, a 135 kDa protein is repeated and both genes are translated and both contain internal repe ated sequences. Deletion mutants in the Imp gene were obtained by cult ivation of M. hominis PG21 with MAb 552 specific for the repeated part of Lmp. One of the Imp genes had deletions of from four to eight repe ats. The other gene was left unaltered. The genes encoding P50/Vaa sho w a different form of variability where domains of the genes seem to b e exchangeable. The genomic maps of five M. hominis strains showed tha t even though the size of the genomes varied the position of the diffe rent genes were in general conserved.