Highly plastic chromosomal organization in Salmonella typhi

Area of Science:

• Microbiology
• Genomics
• Bacterial Genetics

Background:

• Gene order is generally conserved across bacterial genera, including Escherichia coli and Salmonella, despite significant evolutionary divergence.
• Salmonella typhi, the pathogen causing typhoid fever, presents an exception with evidence of chromosomal rearrangements.

Purpose of the Study:

• To investigate the extent and mechanisms of chromosomal DNA rearrangement in Salmonella typhi.
• To explore the role of homologous recombination between ribosomal RNA (rrn) genes in these rearrangements.
• To assess the influence of gene dosage and genomic balance on gene order conservation in S. typhi.

Main Methods:

• Partial digestion of Salmonella typhi chromosomal DNA using the endonuclease I-CeuI.
• Separation of DNA fragments via pulsed-field gel electrophoresis.
• Analysis of fragment order and identification of rearrangements in wild-type strains.

Main Results:

• Twenty-one distinct chromosomal fragment orders were identified among 127 wild-type S. typhi isolates.
• Homologous recombination between the seven rrn genes drives chromosomal rearrangements, including transpositions and inversions.
• Transpositions maintaining proximity to the origin of replication (oriC) were common, while those significantly altering distance were rare.

Conclusions:

• Gene order in S. typhi is less conserved than in related enteric bacteria, primarily due to recombination between rrn genes.
• The gene dosage hypothesis provides a potential explanation for the observed patterns of transposition, suggesting selection against significant changes in gene location relative to oriC.
• Further research is needed to fully elucidate the roles of gene dosage and genomic balance in maintaining bacterial genome structure.