Phylogenomics reveals a diverse Rickettsiales type IV secretion system

Area of Science:

• Microbiology
• Genomics
• Molecular Biology

Background:

• Rickettsiales, an order of Alphaproteobacteria, are obligate intracellular bacteria with reduced genomes, heavily reliant on host resources.
• Transport and secretion systems are crucial for Rickettsiales, with the type IV secretion system (T4SS) being a key area of research.
• The Rickettsiales vir homolog (rvh) T4SS shares homology with the Agrobacterium tumefaciens Vir T4SS but exhibits unique gene duplication and scattered genomic distribution.

Purpose of the Study:

• To investigate the evolutionary history and genomic organization of the rvh T4SS in Rickettsiales.
• To identify conserved and lineage-specific components of the rvh T4SS.
• To explore the potential functions and implications of the rvh T4SS, including its role in host adaptation and potential as a vaccine target.

Main Methods:

• Phylogenetic analysis to estimate the origin of the rvh T4SS.
• Bioinformatic analysis of over 30 Rickettsiales genome sequences.
• Comparative genomics to identify conserved and variable T4SS components.

Main Results:

• Phylogeny suggests a single ancestral acquisition of the rvh T4SS, likely from a non-alphaproteobacterial source.
• A conserved core rvh scaffold, lacking a virB5 homolog, was identified across Rickettsiales genomes.
• Lineage-specific diversification of components like rvhB1, rvhB2, and rvhB9b was observed, correlating with cell envelope modifications.

Conclusions:

• The rvh T4SS has undergone coevolution with Rickettsiales cell envelope morphology, driven by host cell adaptations.
• The rvh T4SS represents a promising target for vaccine development against Rickettsiales infections.
• While effector translocation is the favored function, roles in DNA uptake/release or conjugation remain possible, pending further investigation.