Molecular characterization of the Mycoplasma gallisepticum pvpA gene whichencodes a putative variable cytadhesin protein

A putative cytadhesin-related protein (PvpA) undergoing variation in its ex pression was identified in the avian pathogen Mycoplasma gallisepticum. The pvpA gene was cloned, expressed in Escherichia coli, and sequenced. It exh ibits 54 and 52% homology with the P30 and P32 cytadhesin proteins of the h uman pathogens Mycoplasma pneumoniae and Mycoplasma genitalium, respectivel y. In addition, 50% homology was found with the MGC2 cytadhesin of ill. gal lisepticum and 49% homology was found with a stretch of 205 amino acids of the cytadherence accessory protein HMW3 of M, pneumoniae. The PvpA molecule possesses a proline-rich carboxyterminal region (28%) containing two ident ical directly repeated sequences of 52 amino acids and a tetrapeptide motif (Pro-Arg-Pro-X) which is repeated 14 times. Genetic analysis of several cl onal isolates representing different expression states of the PvpA product ruled out chromosomal rearrangement as the mechanism for PvpA phase variati on. The molecular basis of PvpA variation was revealed in a short tract of repeated GAA codons, encoding five successive glutamate resides, located in the N-terminal region and subject to frequent mutation generating an in-fr ame UAA stop codon, Size variation of the PvpA protein was observed among d l. gollisepticum strains, ranging from 48 to 55 kDa and caused by several t ypes of deletions occurring at the PvpA C-terminal end and within the two d irectly repeated sequences, By immunoelectron microscopy, the PvpA protein was localized on the mycoplasma cell surface, In particular on the terminal tip structure. Collectively, these findings suggest that PvpA is a newly i dentified variable surface cytadhesin protein of M. gallisepticum.