A protein similar to the previously characterized variable surface-exposed membrane protein P120 was identified (P120′), establishing that PG21 possesses a novel gene family. The gene, ', was sequenced and found to have some distinctive properties including a putative start codon of GTG, rather than the common ATG codon, and a coding region with a high G+C content, characteristic of essential housekeeping genes in mycoplasmas. No sequence homology was found to known proteins. The genomic locations of the and ' genes were determined on the restriction map of five strains by PFGE. The genes were localized in two separate regions separated by more than 6 kb. Genes as well as proteins corresponding to P120′ were identified in 24/24 isolates tested and no size variation was detected. 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 characteristics upon Triton X-114 extraction. The N-terminal part of P120′ had a hydrophobic leader sequence without the characteristics of a prolipoprotein. This might explain the membrane association of the protein. Unlike P120, which is frequently recognized by sera of patients seropositive for , P120′ was only rarely recognized. The conserved nature of the P120 gene family indicates that it has an essential, although currently unknown, function.


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