Evolution of exceptionally large genes in prokaryotes.

Area of Science:

• Genomics
• Molecular Biology
• Evolutionary Biology

Background:

• Bacterial genes average approximately 1 kb, but exceptionally large genes (>10 kb) exist.
• The prevalence, function, and origin of these exceptionally large genes (ELSGs) in prokaryotes are not well understood.

Purpose of the Study:

• To investigate the prevalence, potential functions, and evolutionary origins of exceptionally large genes (ELSGs) in prokaryotic genomes.
• To analyze the characteristics and evolutionary mechanisms of ELSGs across diverse bacterial and archaeal species.

Main Methods:

• Searched over 170,000 genes across 46 bacterial and 11 archaeal species to identify ELSGs (>10 kb).
• Performed homology analysis, codon usage bias analysis, and examined evidence of gene expression.
• Investigated evolutionary mechanisms including domain duplication, gene recombination, gene fusion, and horizontal gene transfer.

Main Results:

• Identified 42 ELSGs (0.03% of total genes) in diverse prokaryotic species.
• ELSGs were found to encode nonribosomal peptide synthesis enzymes and, notably, cell surface proteins.
• Intragenic domain duplication and gene recombination were identified as primary drivers of ELSG expansion, with limited evidence of gene fusion.
• Some ELSGs exhibited codon usage bias, evidence of gene expression, and signatures of recent horizontal gene transfer.

Conclusions:

• Exceptionally large genes (ELSGs) are a common feature in prokaryotic genomes, despite their low frequency.
• The evolutionary pathways leading to ELSGs are diverse, involving multiple mechanisms for gene size expansion and acquisition.
• ELSGs contribute to prokaryotic diversity, potentially by encoding complex functional proteins like those involved in cell surface interactions.