brt Gene Review Notes

Colleague Question

Contact: jbanfield@berkeley.edu
Key Interest: Diversity-generating retroelements (DGRs) in phage adaptation

Key Findings

Discovery of Targeted Mutagenesis System

• First DGR system discovered in Bordetella phage BPP-1
• Generates 10^13 protein variants per locus
• Targeted hypermutation for rapid adaptation
• Revolutionary mechanism for protein diversification

Molecular Mechanism

1. Reverse transcription:
2. Template repeat (TR) transcribed to RNA
3. brt reverse transcribes with adenine-specific infidelity

4. A→N mutagenesis (A can become any nucleotide)

5. Targeting specificity:

6. Variable repeat (VR) in target gene replaced
7. Maintains reading frame during mutagenesis

8. Focuses variation on surface-exposed residues

9. Accessory factors:

10. Avd protein essential for retrohoming
11. Forms complex with brt
12. Directs cDNA to specific VR location

Biological Significance

• Phage adaptation: Rapidly evolves receptor binding proteins
• Host range switching: Generates tropism variants
• Immune evasion: Escapes host recognition
• Biofilm targeting: Adapts to different exopolysaccharides

GO Annotation Review

• Added RNA-directed DNA polymerase activity
• Added mutagenic DNA synthesis
• Emphasized retroelement-specific functions
• Distinguished from standard reverse transcriptases

DGR Distribution

• Found in phages, bacteria, archaea

• 1000 DGR systems identified

• Present in human microbiome
• Used by pathogens and commensals

Structural Insights

• PMID:30824605 - Crystal structure of brt
• Unique thumb domain for template jumping
• Conserved RT palm and fingers
• Novel accessory factor binding site

Applications

• Protein engineering tool
• Directed evolution systems
• Antibody diversification
• Novel therapeutic development

Key Publications

• PMID:15184677 - Original DGR discovery
• PMID:25992859 - Mechanism of mutagenic homing
• PMID:30824605 - Structural basis of function
• PMID:32817447 - DGRs across tree of life

Remaining Questions

• How is mutagenesis limited to adenines?
• Can DGRs be engineered for other specificities?
• What prevents catastrophic genome-wide mutagenesis?
• How do DGRs evolve target specificity?

Biotechnology Potential

• Continuous evolution platforms
• Antibody affinity maturation
• Enzyme optimization
• Therapeutic protein development