Circularization pathway of a bacterial group II intron

Area of Science:

• Molecular Biology
• RNA Biology
• Biochemistry

Background:

• Group II introns are catalytic RNAs that splice via branching, circularization, or hydrolysis.
• Circularization, a less-studied pathway, was previously observed to produce intron circles with an extra nucleotide.

Purpose of the Study:

• To investigate the in vivo circularization pathway of bacterial group II introns.
• To identify the origin of the extra nucleotide found in circularized introns.

Main Methods:

• Utilized the Ll.LtrB-ΔA mutant of Lactococcus lactis group II intron in vivo.
• Analyzed intron circles and intermediates to determine splice junction composition and identify exon 2 incorporation.

Main Results:

• Demonstrated that the extra C residue in intron circles originates from the first nucleotide of exon 2.
• Observed accumulation of circularization intermediates containing the first 2-3 nucleotides of exon 2, indicating 3' splice site misrecognition.
• Detected accurate exon ligation in a small fraction of circularized products.

Conclusions:

• Provided the first detailed molecular analysis of group II intron circularization in bacteria.
• Established that branch point removal leads to 3' splice site misrecognition during circularization.
• Suggests that intron circularization is a conserved splicing pathway in bacteria.