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Allelic polymorphism of emm loci provides evidence for horizontal gene spread in group A streptococci
1Department of Epidemiology and Public Health (Microbiology Section), Yale University School of Medicine, New Haven, CT 06510.
Abstract:
Group A streptococci have a virulence regulon containing a single emm locus or two or three distinct and adjacent loci of structurally related emm family genes. The products of the emm gene cluster consist of fibrillar surface proteins, at least some of which are known to contain determinants of type specificity located in their NH2-terminal regions, lying distal to the cell surface. The emm genes can be categorized into four major subfamilies (SFs), based on structural differences within their 3' regions encoding the peptidoglycan-spanning domain. In this study, we investigate the polymorphism within the 5' region of SF-4 and SF-3 emm genes (which occupy the first and last emm positions of the gene cluster, respectively) in 22 strains representing different serotypes. Our findings indicate that unlike the centrally positioned SF-1 or SF-2 genes, SF-3 and SF-4 genes each display only limited polymorphism in their 5' regions, suggesting that their gene products may not be major contributors to type specificity. Two forms of the SF-3 gene (SF3a, SF3b) and two forms of the SF-4 gene (SF4a, SF4b) are found to exist in all four possible combinations (SF3aSF4a, SF3aSF4b, SF3bSF4a, SF3bSF4b), strongly suggesting that horizontal gene spread has contributed to the evolution of emm genes and to the generation of emm gene diversity in group A streptococci.
Insights
Group A streptococci emm genes SF-3 and SF-4 show limited 5' region polymorphism, suggesting they don't determine type specificity. Horizontal gene spread likely drives emm gene diversity in these bacteria.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Group A streptococci possess virulence regulons with emm gene clusters encoding surface proteins.
- emm genes are categorized into four subfamilies (SFs) based on 3' region variations.
- Type specificity determinants are often located in the NH2-terminal regions of emm gene products.
Purpose of the Study:
- To investigate polymorphism in the 5' regions of SF-3 and SF-4 emm genes.
- To understand the contribution of SF-3 and SF-4 genes to type specificity.
- To explore the evolutionary mechanisms behind emm gene diversity.
Main Methods:
- Analysis of emm gene polymorphism in the 5' region.
- Studied 22 strains of Group A streptococci with diverse serotypes.
- Compared polymorphism in SF-3 and SF-4 genes with centrally positioned SF-1 and SF-2 genes.
Main Results:
- SF-3 and SF-4 emm genes exhibit limited 5' region polymorphism compared to central genes.
- Two forms of SF-3 (SF3a, SF3b) and two forms of SF-4 (SF4a, SF4b) were identified.
- All four combinations of SF-3 and SF-4 gene forms (SF3aSF4a, SF3aSF4b, SF3bSF4a, SF3bSF4b) were observed across strains.
Conclusions:
- The limited polymorphism suggests SF-3 and SF-4 gene products may not be primary determinants of type specificity.
- The existence of multiple combinations of SF-3 and SF-4 variants strongly indicates horizontal gene spread.
- Horizontal gene transfer is a significant factor in the evolution and diversity of emm genes in Group A streptococci.