The vsa genes of Mycoplasma pulmonis encode the V-1 lipoproteins. Most V-1 proteins contain repetitive domains and are thought to be involved in mycoplasma-host cell interactions. Previously, we have reported the isolation and characterization of six vsa genes comprising a 10-kb region of the genome of M. pulmonis strain KD735-15. In the current study, vsa-specific probes were used to clone several fragments from a genomic library of KD735-15 DNA and assemble a single 20-kb contig containing 11 vsa genes. The middle region of the vsa locus contains a large open reading frame (ORF) that is not a vsa gene and has undergone an internal deletion in some strains. The ORF is predicted to encode a membrane protein that may have a role in disease pathogenesis. To examine vsa genes in a strain of M. pulmonis that is unrelated to KD735-15, strain CT was studied. Through Southern hybridization and genomic cloning analyses, CT was found to possess homologs of the KD735-15 vsaA, -C, -E, and -F genes and two unique genes (vsaG and vsaH) that were not found in KD735-15. High-frequency, site-specific DNA inversions serve to regulate the phase-variable production of individual V-1 proteins. As a result of the sequence analysis of vsa recombination products, a model in which DNA inversion arises from strand exchange involving at least six nucleotides of the vrs box is proposed.Mycoplasmas cause slowly progressive, chronic diseases in human and animals. The mechanisms of mycoplasmal disease pathogenesis are poorly understood, and there are no effective control measures. Mycoplasma pulmonis is the etiologic agent of murine respiratory mycoplasmosis and can also cause genital disease and arthritis in rats and mice (31). Thus, M. pulmonis can colonize a variety of epithelial surfaces. Rat isolates of M. pulmonis such as strains UAB 5782 and UAB 6510 are generally more virulent in rats than in mice (10,11,24). In the mouse, UAB 5782 and UAB 6510 colonize the respiratory tract without usually causing lesions (10). In contrast, the mouse isolate strain CT causes severe respiratory disease in the mouse (6,7,10,12). Mycoplasma factors that contribute to the host specificity of disease are unknown. A comparison of the proteins produced by 18 strains of M. pulmonis revealed mostly conserved proteins that were invariant among strains (38). An exception was the V-1 family of surface proteins that are encoded by the vsa (variable surface antigen) genes (4,21,33,35,39). Variation in the V-1 proteins may contribute to the host specificity of the mycoplasma and to the chronicity and severity of disease.The chronic nature of mycoplasmal diseases indicates that mycoplasmas can adapt to the rapidly changing conditions in the host. Previous studies had shown that phenotypic variation and genetic recombination occur at high frequencies in M. pulmonis (3). The vsa genes comprise one of the highly recombinogenic loci in this species. Recombination between vsa genes involves site-specific DNA inversions occurring at a 34-bp sequence that defines the vsa ...
Mice carrying a gamma2b transgene have been shown previously to be deficient in B cell development. In particular, a developmental block exists at the pre-B cell stage. The few B cells that develop all express endogenous micro heavy chains. The phenotype suggests that gamma2b exerts a strong feedback inhibition on endogenous Ig gene rearrangement, but, unlike micro, cannot support further B cell development. In this study we have created hybrid transgenes between gamma2b and micro. Transgenic mice with a C(H)1 domain of micro, or both a C(H)1 and transmembrane/cytoplasmic domain of micro replacing the respective domains of a gamma2b transgene, have the same B cell defect as gamma2b transgenic mice. Interestingly, the severity of the defect is correlated with the level of expression of the transgene, suggesting that the degree of feedback inhibition of Ig gene rearrangement depends on the level and timing of Ig production. Crossing the gamma2b/micro transgenes into a Bcl-x(L) transgenic line allows immature gamma2b B cells to survive, but not to develop to maturity. Therefore, the missing function of micro is not simply an anti-apoptotic effect.
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