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Localized reversible frameshift mutation in an adhesin gene confers a phase-variable adherence phenotype in
1Department of Molecular Microbiology and Immunology, University of Missouri School of Medicine, Columbia 65212, USA.
Abstract:
The variable adherence-associated (Vaa) antigen of Mycoplasma hominis is an abundant surface lipoprotein adhesin that may mediate important interactions of this wall-less prokaryotic pathogen with the human host. Extensive mutational variation of Vaa size, as well as sequence and antigenic divergence, has been described previously. Using a series of clonal isolates representing an isogenic lineage of variants oscillating in Vaa expression, Vaa is further shown in this study to undergo high-frequency phase variation in expression, which correlated precisely with the ability of M. hominis to adhere to cultured human cells. Although no DNA rearrangements or sequence differences in the 5' regions flanking vaa alleles were detected between Vaa+ and Vaa variants, intragenic vaa sequences from this lineage revealed an oscillating mutation involving a single nucleotide deletion/insertion in a short tract of adenine residues near the 5' end of the mature Vaa coding sequence, which created a translational frameshift resulting in either a complete Vaa ORF or an in-frame UAG stop codon immediately downstream of the poly-A tract. Evidence for the occurrence of this high-frequency frameshift mutation in vivo was obtained from analysis of PCR-generated vaa sequences amplified from the joint synovial fluid of a patient with M. hominis-associated arthritis, which indicated that Vaa phase variation occurs during M. hominis infection in the natural host. These results identify a distinctive frameshift mutator element in the vaa gene that governs M. hominis adherence and highlight the importance of mutational alteration of primary gene products on the mycoplasma surface as a means of generating and maintaining functional diversity in the host.
Insights
Mycoplasma hominis uses phase variation in its variable adherence-associated (Vaa) antigen expression to control adherence to human cells. This variation, driven by a frameshift mutation, occurs during infection, enabling pathogen diversity.
Area of Science:
- Microbiology
- Molecular Biology
- Pathogenesis
Background:
- Mycoplasma hominis is a wall-less bacterial pathogen.
- The variable adherence-associated (Vaa) antigen is a surface lipoprotein involved in host interactions.
- Previous studies noted Vaa antigen size and sequence variation.
Purpose of the Study:
- To investigate the mechanism and significance of Vaa antigen expression variation in Mycoplasma hominis.
- To determine if Vaa expression phase variation correlates with bacterial adherence.
- To explore the in vivo occurrence of Vaa phase variation during infection.
Main Methods:
- Analysis of clonal isolates with oscillating Vaa expression.
- Correlation of Vaa expression levels with adherence to cultured human cells.
- DNA sequencing of vaa gene regions to identify mutations.
- PCR amplification and sequencing of vaa from patient synovial fluid.
Main Results:
- High-frequency phase variation in Vaa antigen expression was observed.
- Vaa expression directly correlated with M. hominis adherence to host cells.
- A frameshift mutation within a poly-adenine tract in the vaa gene was identified as the cause of phase variation.
- Evidence of Vaa phase variation was found in synovial fluid from an arthritis patient, indicating in vivo occurrence.
Conclusions:
- A frameshift mutator element in the vaa gene regulates Mycoplasma hominis adherence.
- Phase variation of surface antigens is a key mechanism for generating functional diversity in mycoplasma.
- Vaa phase variation occurs during natural infection, impacting host-pathogen interactions.