MYCOPLASMA-HOMINIS EXPRESSES 2 VARIANTS OF A CELL-SURFACE PROTEIN, ONE A LIPOPROTEIN, AND ONE NOT

A protein similar to the previously characterized variable surface-exp osed membrane protein P120 was identified (P120'), establishing that M ycoplasma hominis PG21 possesses a novel gene family. The gene, p120', was sequenced and found to have some distinctive properties including a putative start codon of G+G, rather than the common ATC codon, and a coding region with a high G+C content, characteristic of essential h ousekeeping genes in mycoplasmas. No sequence homology was found to kn own proteins. The genomic locations of the p120 and p120' genes were d etermined on the restriction map of five M. hominis strains by PFGE. T he genes were localized in two separate regions separated by more than 6 kb. Genes as well as proteins corresponding to P120' were identifie d in 24/24 M. hominis isolates tested and no size variation was detect ed. P120' had a molecular mass of 98 kDa, 20 kDa smaller than P120 as estimated by SDS-PAGE. The protein was surface-exposed and associated with the mycoplasma membrane, but had predominantly hydrophilic charac teristics upon Triton X-114 extraction. The N-terminal part of P120' h ad a hydrophobic leader sequence without the characteristics of a prol ipoprotein. This might explain the membrane association of the protein . Unlike P120, which is frequently recognized by sera of patients sero positive for M. hominis, P120' was only rarely recognized. The conserv ed nature of the P120 gene family indicates that it has an essential, although currently unknown, function.