Group II introns are large catalytic RNAs (ribozymes) and retrotransposable elements of bacterial origin. They invade genomes by ‘retrohoming', a highly site-specific mobility pathway that is operated both by the intron ribozyme and the intron-encoded reverse transcriptase. Most often, mobile group II introns insert themselves into intergenic regions of the genome, which is consistent with the ‘selfish' behavior of these mobile elements. In contrast with this trend however, some classes of group II introns target particular structures or genetic contexts. This is the case of some particular group II introns that are found associated with 'start' or 'stop' codons of specific genes. These 'specialized' introns, abundant in Gram-negative bacteria, also share peculiar structural features. To study the biology of these retrotransposable elements we are using as a model system an E. coli group II intron retrotransposon whose natural integration site corresponds to the ‘stop' codon of the heat-shock gene groEL. Using genetic and molecular approaches, we have shown that this 'groEL' intron is a highly mobile and site-specific retrotransposon. Interestingly, we have also demonstrated that the ‘groEL' element can invade an ectopic copy of its target site, engineered in a transcriptionally silent locus of the host chromosome, as efficiently as the authentic target site at the end of the groEL ORF. To further dissect the mobility mechanism of this retrotransposon, we are currently testing the importance of a specific base-pairing interaction between the retrotransposon RNA and its DNA target on the ability of the ‘groEL' element to integrate into its target site. In parallel, we have constructed several E.coli laboratory strains carrying a genomic copy of either the complete retrotransposon or different structural variants of it. These strains are being characterized phenotypically in order to uncover a potential role for the ‘groEL' element in the physiology of the host cell.