A new beginning with new ends: linearisation of circular chromosomes during bacterial evolution

Area of Science:

• Microbiology
• Genetics
• Evolutionary Biology

Background:

• Bacterial chromosomes are typically circular, but some species have evolved linear chromosomes.
• Linear chromosomes, observed in Borrelia burgdorferi and Streptomyces, may originate from linear plasmids.
• These linear chromosomes retain a central origin of replication from their circular ancestors and employ plasmid strategies to solve replication end-binding problems.

Purpose of the Study:

• To investigate the evolutionary origins and structural characteristics of linear bacterial chromosomes.
• To understand the mechanisms of genetic instability and plasticity associated with linear chromosomes in bacteria.
• To explore the reversibility of chromosome linearity in bacteria, specifically in Streptomyces.

Main Methods:

• Comparative genomics analysis of bacterial species with linear chromosomes.
• Investigation of genetic instability through observation of amplifications and deletions.
• Experimental manipulation to induce circularization of linear chromosomes and vice versa.

Main Results:

• Streptomyces linear chromosomes exhibit high instability, with frequent amplifications and deletions, often affecting telomeres.
• Chromosome linearity in Streptomyces is reversible, with spontaneous or induced formation of circular chromosomes.
• Genetic instability is not exclusive to linear chromosomes; circularized Streptomyces chromosomes are also unstable.
• Linear chromosomes may possess telomere-linked regions of increased genomic plasticity, facilitating genetic exchanges and rearrangements.

Conclusions:

• Bacterial linear chromosomes represent a sporadic evolutionary event, potentially arising from linear plasmids.
• The genetic instability observed in linear chromosomes, particularly in Streptomyces, is a significant feature.
• The reversibility of chromosome linearity suggests dynamic evolutionary pathways in bacteria.
• Telomere-linked genomic plasticity may contribute to the differential evolution of genes based on chromosomal location.