A SURFACE EPITOPE UNDERGOING HIGH-FREQUENCY PHASE VARIATION IS SHAREDBY MYCOPLASMA-GALLISEPTICUM AND MYCOPLASMA-BOVIS

We have recently reported that three distinct size- and phase-variable surface lipoproteins (Vsps) of the bovine pathogen Mycoplasma bovis p ossess a common epitope recognized by monoclonal antibody 1E5. In the present study, we show that this epitope is also present on a size-var iant protein (PvpA) of the avian pathogen Mycoplasma gallisepticum. Ap plication of monoclonal antibody 1E5 in Western immunoblot analysis of Triton X-114 phase-fractionated proteins and in colony immunoblots, a s well as in trypsin and carboxypeptidase digestion experiments, has d emonstrated that (i) PvpA is an integral membrane protein with a free C terminus, (ii) the shared epitope is surface exposed, and (iii) PvpA is subjected to high-frequency phase variation in expression. By usin g serum antibodies from M. gallisepticum-infected chickens, we were ab le to demonstrate the immunogenic nature of PvpA and identify three ad ditional highly immunogenic Triton X-114 phase proteins (p67, p72, and p75) also undergoing high-frequency phase variation spontaneously and independently. Metabolic labeling experiments with [C-14]palmitate an d [C-14]oleate revealed that PvpA, in contrast to p67, p72, and p75, i s not lipid modified. Southern blot hybridization with restriction fra gments carrying the pvpA gene of M. gallisepticum or the vspA gene of M. bovis against digested genomic DNA of the two Mycoplasma species in dicated the absence of genetic relatedness between the pvpA and vspA g enes. The apparent complexity of the antigenic variation phenomenon in M. gallisepticum is discussed.