Current Bacterial Gene Encoding Capsule Biosynthesis Protein CapI Contains Nucleotides Derived from Exonization

Area of Science:

• Genomics
• Molecular Evolution
• Bioinformatics

Background:

• The introns-early hypothesis posits that introns were present in early life forms, including bacterial ancestors.
• Most eukaryotic genomes are intron-rich, but direct evidence for introns in ancestral bacteria remains elusive.
• Current bacterial genomes lack introns in protein-coding genes, making ancestral intron presence difficult to ascertain.

Purpose of the Study:

• To investigate the potential existence of introns in ancestral bacterial genomes.
• To analyze horizontally transferred bacterial genes in eukaryotes for evidence of ancient introns.
• To contribute to the debate between the introns-early and introns-late theories.

Main Methods:

• Sequence analysis of the bacterial capsule biosynthesis protein CapI gene in the sea anemone Nematostella vectensis.
• Comparative genomics to assess sequence identity between the identified intron and coding regions of current bacterial CapI genes.
• Splice site analysis to infer the mechanism of intron loss in bacterial genomes.

Main Results:

• A 40-base pair phase 1 intron was identified within the horizontally transferred bacterial CapI gene in Nematostella vectensis.
• High sequence identity between the intron and flanking exon sequences suggests a common ancestral origin.
• Splice site characteristics indicate intron loss via deletion mutation and subsequent exonization in extant bacterial CapI genes.

Conclusions:

• The findings suggest that the bacterial CapI gene contained spliceosomal introns in ancient times.
• This represents the first sequence-based evidence for introns in ancestral bacterial genes.
• The employed methodology can potentially uncover further evidence for introns in early bacterial evolution, aiding the introns-early/introns-late debate.