Role of Class C Group II Introns in the Formation of Mobile Gene Cassettes in Integrons

CECILIA QUIROGA; GREGORY LEÓN; DANIELA CENTRÓN; PAUL H. ROY

Magog, Quebec, Canadá

Congreso; 6th Ribo-Club Opening Session; 2005

Integrons are elements that use an integron integrase, a tyrosine recombinase, to incorporate gene cassettes into a specific attI site.  Cassettes usually consist of a single, promoterless gene accompanied by an attC site, a palindromic sequence of 60-140 bp.  The integrase integrates and excises cassettes by attI x attC and attC x attC recombination. In plasmid-borne integrons, where cassettes are mostly antibiotic resistance genes, analysis indicates that the structural genes and their attC sites have evolved separately before being assembled. A major unanswered question is how structural genes become associated with attC sites to form cassettes. We have found several examples, including a Serratia marcescens plasmid, where class C group II introns occur at the junction of a structural gene and an attC site in integrons.  These structural gene-group II intron-attC site assemblies are potential intermediates in cassette formation. We have found potential earlier intermediates, structural gene-group II intron and group II intron-attC site, in Nitrosomonas europaea, Geobacter sulfurreducens, and several other bacterial chromosomes. We are investigating how group II introns may capture attC sites (probably by retrohoming to the left end of attC sites, which resemble transcription terminators) and structural genes (possibly by the less specific mechanism of ectopic insertion) and recombine to assemble them.  We have demonstrated group II intron mobility in the S. marcescens system , and splicing in both the S. marcescens and N. europaea systems.