INCREASED STRUCTURAL AND COMBINATORIAL DIVERSITY IN AN EXTENDED FAMILY OF GENES ENCODING VLP SURFACE-PROTEINS OF MYCOPLASMA-HYORHINIS

Variable lipoproteins (Vlp) constitute the major coat protein of Mycop lasma hyorhinis. They are products of multiple, divergent, single-copy genes organized in a chromosomal cluster, Three genes, vlpA, vlpB, an d vlpC, have been previously identified in clonal isolates of M. hyorh inis SK76. Each is linked to a characteristic promoter region containi ng a homopolymeric tract of adenine residues [poly(A) tract], subject to hypermutation that transcriptionally controls phase variation of vi p genes and leads to combinatorial surface mosaics of distinct Vlp pro ducts, The size of the natural rip gene repertoire is unknown but may critically determine the degree of structural and combinatorial divers ity available in this species, In this study, the vip repertoire of M. hyorhinis GDL-1 was characterized and shown to contain three addition al genes, vlpD, vlpE, and vlpF, clustered with other known vlp genes i n the order 5'-vlpD-vlpE-vlpF-IS-vlpA-IS-vlpB-vlpC-3' where IS represe nts copies of the IS1221 element of M. hyorhinis. The 5' boundary of t his expanded family was identical to that of the more limited family 5 '-vlpA-IS-vlpB-vlpC-3' previously described in a clonal isolate of str ain SK76, A recombinant construct containing vlpD, vlpE, and vlpF expr essed antigenically distinguishable products corresponding to each gen e, These genes encode characteristic C-terminal repetitive regions tha t are subject to size variation by insertion or deletion of intragenic repeats but maintain an extended, charged structure, Each vip gene al so contained characteristic alternative open reading frames, which pro vide a potential reservoir of coding sequence for Vip diversity, possi bly recruited through insertion and/or deletion mutations. These findi ngs demonstrate a vastly expanded potential for structural diversity a nd combinatorial display of surface mosaics on this organism and sugge st that modulation of the vip repertoire, possibly in conjunction with mobile elements, may determine the capacity for surface variation in natural populations and laboratory strains of this mycoplasma species.