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Infection and Immunity, April 2002, p. 2220-2225, Vol. 70, No. 4
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.4.2220-2225.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Extended Repertoire of Genes Encoding Variable Surface Lipoproteins in Mycoplasma bovis Strains

Sarit Nussbaum,1 Inessa Lysnyansky,1 Konrad Sachse,2 Sharon Levisohn,3 and David Yogev1*

Department of Membrane and Ultrastructure Research, The Hebrew University—Hadassah Medical School, Jerusalem 91120,1 Mycoplasma Unit, Kimron Veterinary Institute, Bet Dagan 50250, Israel,3 Federal Institute for Health Protection of Consumersand Veterinary Medicine, Division 4, Jena, Germany2

Received 10 August 2001/ Returned for modification 4 December 2001/ Accepted 8 January 2002


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ABSTRACT
 
A genomic cluster of vsp genes was previously shown to mediate high-frequency phenotypic switching of surface lipoprotein antigens in the bovine pathogen Mycoplasma bovis. This study revealed that field strains of M. bovis possess modified versions of the vsp gene complex in which extensive sequence variations occur primarily in the reiterated coding sequences of the vsp structural genes. These findings demonstrate that there is a vastly expanded potential for antigenic variation within populations of this organism.


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TEXT
 
An important microbial strategy for host adaptation and evasion of immune responses is the maintenance of diversity in propagating populations. This is commonly manifest in structurally variant forms of major coat proteins that are transiently expressed as a result of random or programmed molecular events (4, 6, 22, 23, 27, 28). In many cases, large families of related genes organized in genomic loci are utilized. Homologous recombination, gene conversions, gene duplications, additions or deletions of tandem repetitive units, and DNA inversions are mechanisms that are frequently used to regulate the phase-variable expression of these genes at a high frequency (4, 6, 11, 12, 13, 14, 15, 19, 22, 23, 30).

Mycoplasma bovis, a bovine pathogen (8, 16, 25), contains an elaborate genetic system in which multiple related but divergent genes encoding variable surface lipoproteins (Vsps) undergo spontaneous on/off switching (9, 10). The Vsp antigens have been shown to be highly immunogenic and to contain adhesive structures in the repetitive domains of their molecules (1, 21). High-frequency site-specific DNA inversions that occur within the vsp locus dictate the degree of Vsp diversification within propagating populations by determining which vsp gene is expressed in a given cell (9, 12). The extent of Vsp antigenic variation also depends strongly on the number of vsp genes available, particularly with respect to the generation of combinatorial diversity. Recently, we have shown that the vsp gene repertoire undergoes changes by intrachromosomal recombination within the vsp locus between closely related vsp genes, which leads to the generation of a chimeric vsp gene (11). The extent of the natural vsp gene reservoir in M. bovis is not known, but its size and modulation may significantly affect the adaptive ability of this species. The present study was undertaken to examine the extents of the vsp gene repertoire in different field strains of M. bovis.

Eight strains of M. bovis isolated from distinct sites (lung, joint, trachea, nose, and milk) in bovine hosts with various clinical presentations (mastitis, pneumonia, arthritis, and healthy) (20) were examined. Genomic DNAs of these strains were extracted, digested with the HindIII restriction enzyme, and subjected to Southern blot hybridization under low-stringency conditions. A 1.5-kb HindIII fragment carrying the vspA structural gene and its highly conserved vsp upstream region (10) and two oligonucleotides representing unique repetitive coding sequences present only in the vspE gene (RE1) or only in the vspF gene (RF2) of type strain PG45 (10) were used as probes. Complex hybridization profiles for multiple HindIII genomic fragments with different intensities and of different sizes were obtained for the strains examined with the vspA gene probe (Fig. 1A). The variations in the vsp-related fragments among strains (2, 17) presumably reflect the presence of distinct configurations of the vsp genomic locus due to high-frequency site-specific DNA inversions, which have been shown to occur in that locus (11, 12).



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  		FIG. 1. Identification of multiple vsp-related genomic fragments in M. bovis strains. Four micrograms of chromosomal DNA of each strain was digested with the HindIII restriction enzyme, subjected to Southern blot hybridization, and probed with the 32P-labeled vspA gene (A), with oligonucleotide RE1 (B), and with oligonucleotide RF2 (C). The conditions used for oligonucleotide labeling, DNA labeling, and Southern blot hybridization have been described elsewhere (9, 10). The M. bovis strains used included type strain PG45 (lane 1), D490 (lane 2), D857 (lane 3), D860 (lane 4), D1098 (lane 5), D1499 (lane 6), D1517 (lane 7), and 422 (lane 8). The positions of molecular size markers are indicated on the left.

Interestingly, however, the RE1 region, which is repeated in tandem eight times within the vspE structural gene of the PG45 strain and is localized on a single 1.2-kb HindIII genomic fragment (Fig. 1B, lane 1) (10), was not observed in five different strains (Fig. 1B, lanes 2 to 5 and 8), and in one strain it was identified in several HindIII genomic fragments (Fig. 1B, lane 7). The RF2 region, which is repeated seven times within the vspF structural gene and is localized on a single 1.8-kb HindIII fragment in the PG45 strain (Fig. 1C, lane 1) (10), was missing in one strain (Fig. 1C, lane 8). Collectively, the data suggest that extensive variations in the vsp reiterated coding sequences might occur among strains.

One strain, strain 422, which was isolated from the lung of a calf with pneumonia and in which both the RE1 and RF2 repetitive domains were missing (Fig. 1B and C, lanes 8), was chosen for further analysis. The vsp genomic locus of this strain was cloned and sequenced. A cluster of 11 Vsp-related open reading frames (ORFs) that were not all similarly oriented was identified (Fig. 2). The nucleotide sequences of the 11 vsp genes and the deduced proteins were compared to the nucleotide sequences of the 13 vsp counterparts and the deduced proteins in the PG45 strain (10). The vsp genes of the two strains were found to have two highly homologous domains. One of these domains is a 5' upstream region that can be divided into two cassettes. A 72-bp region upstream of the ATG initiation codon contains a putative ribosome binding site and exhibits 99% homology in all vsp genes, while the second cassette, which is upstream of the first cassette, is more divergent (Fig. 3A). The second domain is the N-terminal region encoding 32 amino acids that exhibit 98% identity in the Vsps and contain a typical prokaryotic lipoprotein signal peptide. The protein sequence begins with three positively charged Lys residues, followed by a core of 20 hydrophobic amino acids, and it ends with the tetrapeptide Ala-Ala-Lys-Cys (Fig. 3B).



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  		FIG. 2. Schematic representation, partial restriction map, genomic organization, and comparison of the vsp locus of M. bovis strain 422 and the vsp locus of M. bovis type strain PG45. (A) vsp locus of strain 422. The solid line represents a 16.7-kb M. bovis DNA insert obtained from a recombinant bacteriophage designated {lambda}Mb422-1 isolated from a genomic library constructed with the phage vector {lambda}GEM-12 (XhoI half arms; Promega). The library was screened with the vspA gene as previously described (9, 10). The positions of HindIII (H) and EcoRI (E) restriction sites are indicated. The locations and directions of 11 Vsp ORFs are indicated by labeled arrows. The positions of three non-Vsp-related ORFs (ORF-1 to ORF-3) are also indicated by labeled arrows. The highly homologous vsp upstream regions, as well as the conserved N-terminus-encoding regions, are indicated by the cross-hatched portions of the arrows. The nucleotide sequences of vsp genes 1 to 11 of M. bovis strain 422 have been assigned GenBank accession numbers AF406972 to AF406982, respectively. (B) vsp locus of M. bovis type strain PG45, as previously described (10). The position of the ClaI (C) restriction site is also indicated.



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  		FIG. 3. Sequence alignments for vsp 5' upstream regions (A) and Vsp-N-terminal regions (B). Five representatives of the vsp gene family of M. bovis strain 422 (vsp422-1 to -5) and the vspA gene, a representative vsp gene of M. bovis type strain PG45, were compared. The alignment was constructed with the MacVector 6.0 software program. The numbers above the sequences indicate positions relative to the initiation codon. Nucleotides representing a putative ribosome-binding site (SD) are overlined. The initiation codon (ATG) is indicated by a horizontal arrow. The division of the vsp 5' upstream region into two distinct cassettes is indicated by a vertical arrow at nucleotide position -72 (10). Identical nucleotides or amino acid resides are indicated by shaded boxes. The single Cys residue in the Vsp lipoprotein box is indicated by an arrowhead.

Interestingly, in contrast to the highly conserved nature of the vsp 5' upstream region and of the N-terminal domain, there is considerable divergence in the deduced amino acid sequences of the Vsp proteins between the two strains. In both M. bovis PG45 and 422, most of the Vsp molecules contain an unusual motif consisting of reiterated coding sequences extending from the N terminus to the C terminus of the protein chain that create a periodic polypeptide structure (Fig. 4) (10). The majority of these repeated sequences are arranged in tandem domains that comprise up to 80% of the Vsp molecule. However, the substantial sequence divergence among individual Vsps within each family and the sequence divergence between the two families are the result of at least 30 repetitive units consisting of different amino acid sequences of different lengths (range, 4 to 127 amino acids), which are distributed within the Vsp proteins of the two families (Table 1) (10). Within the vsp gene family of strain 422, there are repeats that are present in only one Vsp (e.g., R1-2, R3-1, R4-2, R5-2, R5-3, R7-1, R9-3, R9-4, R10-2, and R10-3), whereas other repeats occur at variable locations in several Vsps (e.g., R1-1, R3-2, R4-1, R6-1, R6-2, and R10-2) (Fig. 4 and Table 1).



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  		FIG. 4. Structural features and comparison of vsp genes and Vsp products of M. bovis strain 422. The structures of the vsp genes and the predicted Vsp proteins are represented schematically by aligned boxes. A highly conserved region upstream of each vsp gene is represented by the first gray box. The second box (solid box labeled S) represents a highly homologous 75-bp DNA sequence encoding a conserved prolipoprotein signal peptide. A sequence of six amino acids common to all Vsps is represented by the third box (dark gray). In-frame reiterated coding sequences extending from the N termini to the C termini of the Vsp proteins and encoding periodic amino acid sequences are represented by different kinds of boxes. Distinctive repetitive domains within each Vsp are labeled with R and the number of the corresponding vsp gene. Repetitive units present in more than one Vsp molecule are represented similarly. The number to the right of each Vsp indicates the length of the Vsp polypeptide chain.


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  		TABLE 1. Repetitive sequences in M. bovis strain 422 Vsp proteins

Importantly, a comparison of all vsp reiterated coding sequences in the two M. bovis strains revealed the presence of strain-specific repetitive sequences. Nine repeats were found only in strain 422 (R1-2, R4-2, R5-2, R5-3, R7-1, R9-3, R9-4, R10-2, and R10-3, consisting of 6, 77, 5, 10, 127, 16, 16, 10, and 15 amino acids, respectively), and 13 repeats were found only in the PG45 strain (RA-2, RA-3, RA-4, RE-1, RF-1, RF-2, RG-2, RG-4, RJ-1, RK-2, RK-3 RN-2, and RO-2, consisting of 6, 8, 8, 6, 84, 10, 10, 11, 26, 6, 11, 87, and 12 amino acids, respectively) (Table 1) (10). Other repeats exhibited partial homology in the two strains (for example, R3-1), while five repeats were found in both strains (R3-2, R4-1, R6-1, R6-2, and R8-2, consisting of 6, 6, 6, 11, and 8 amino acids, respectively). Notably, however, the distribution of the repeats found in both strains within individual Vsps, as well as their copy numbers, varied considerably between the two vsp gene families (Fig. 4) (10). Together, the presence of strain-specific vsp repetitive coding sequences and the distribution of shared repeats among vsp genes may generate an amplified array of Vsp phenotypes that differ in their antigenic characteristics.

The results of this study revealed expanded possibilities for adaptive surface variation for the vsp gene system. Not only do the phase and size variations among Vsps create extensive potential for antigenic and structural diversification on the surface of M. bovis, but also the extent of the vsp gene repertoire that can be expressed in a population provides an additional factor that may profoundly affect the possible variation. Documented examples of increased repertoires of genes encoding variable antigens include the vlp system of the swine pathogen Mycoplasma hyorhinis and the vsa system of the murine pathogen Mycoplasma pulmonis. In M. hyorhinis the high-passage avirulent GDL strain possesses six vlp genes (vlpA to vlpF), while clonal isolates of the pathogenic strain SK76 contain three or seven vlp genes (vlpA to vlpC or vlpA to vlpG) (5, 29). M. pulmonis strain UAB CTIP contains 7 vsa genes, while M. pulmonis strain KD735-15 contains 11 vsa genes (3, 24). In contrast, with the exception of the conserved Vsp N-terminal region, none of the vsp genes of M. bovis strain PG45 was identical to a vsp gene of M. bovis strain 422. Thus, extended antigenic phenotypes within a propagating population appear to be a selective and heritable feature of these organisms and probably contribute significantly to the successful persistence of chronic infections caused by pathogenic mycoplasmas in their natural hosts (18, 25, 28).

The repetitive surface-exposed C-terminal ends of the Vsp proteins were shown to possess immunogenic epitopes and adhesive structures, suggesting that these molecules may play a role as complex adherence-mediating regions in pathogenesis (21). The finding that variations among individual Vsps of both strains were localized solely within Vsp repetitive regions may suggest that strains of the same species were subjected to different selective pressures by their hosts during evolution. Notably, two ORFs embedded in the vsp locus of strain PG45 exhibit high homology to mobile genetic elements. For example, ORF-1 exhibits homology to insertion sequence IS4 and ORF-2 exhibits homology to insertion sequence IS30 of Escherichia coli (10). The existence of M. bovis strains carrying modified versions of the vsp gene system and the presence of ORFs associated with mobile elements or mobility functions raise an intriguing question about the possibility of natural gene transfer among cells that are in direct contact in propagating populations or even during infection within the natural host (26). The presence of multiple regions with a high level of sequence similarity upstream of the vsp genes and within the vsp coding regions would allow modulation of the vsp gene repertoire by recombination processes (6, 11, 12, 15).

Analyses of genetic systems that mediate antigenic variation in several pathogenic mycoplasmas (3, 7, 12, 14, 24, 30) have revealed diverse mechanistic features of expression and structural variations. The presence of an extended repertoire of genes encoding variable surface antigens in strains of the same species provides another important means of surface diversification and underscores the efficient way that these wall-less minute microorganisms manipulate their limited genetic material for maximum adaptive flexibility in the host.


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ACKNOWLEDGMENTS
 
This study was supported in part by the German-Israeli Foundation for Scientific Research and Development, by research grant IS-3126-99 from the United States-Israel Binational Agricultural Research and Development Fund, and by the Israel Academy of Sciences and Humanities Foundation.


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FOOTNOTES
 
* Corresponding author. Mailing address: Department of Membrane and Ultrastructure Research, The Hebrew University—Hadassah Medical School, Jerusalem 91120, Israel. Phone: 972-2-6758176. Fax: 972-2-6784010. E-mail: yogev{at}cc.huji.ac.il. Back

Editor: V. J. DiRita


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Infection and Immunity, April 2002, p. 2220-2225, Vol. 70, No. 4
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.4.2220-2225.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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