Group II introns and expression of conjugative transfer functions in lactic acid bacteria

The homologous lactococcal conjugative elements pRS01 and the sex factor of Lactococcus lactis strain 712 both contain a Group II intron within a gene believed to encode a conjugative relaxase enzyme. This enzyme is responsible for nicking of DNA at the origin of transfer (oriT) sequence of the sex factor DNA to initiate the strand transfer process. Group II introns have been studied in eukaryotes, and several of these elements in yeast mitochondrial genes have received considerable attention. These introns are relatively large in size and generally encode a protein within the intron sequence. In addition to splicing activity, Group II introns are mobile genetic elements. The intron-encoded proteins (IEPs) contain endonuclease and reverse transcriptase domains believed to play an enzymatic role in genetic mobility reactions, while a putative maturase domain is thought to promote splicing by stabilizing the folding of the intron RNA into an active ribozyme structure which carries out the splicing reaction. The lactococcal introns represent the first examples of Group II introns shown to be functional in vivo in prokaryotes. Because of the advantages of a bacterial system for genetic and molecular studies, the Ll.ltrB intron from pRS01 has attracted the attention of several laboratories interested in Group II intron biology. Recently, it has been shown that the system can be adapted to function in Escherichia coli (although at somewhat reduced efficiency). In addition, it has been recently proven that the best studied form of mobility, the homing of the intron into an intronless allele of the cognate exon gene, occurs via an RNA intermediate and does not require DNA homology or generalized host recombination functions. Current efforts are analysis of the role Ll.ltrB splicing in regulating expression of pRS01 conjugation functions. The lactococcal Group II introns represent the first demonstrated genetically mobile prokaryotic retroelements, and they also have considerable potential as genetic engineering tools for Lactic Acid Bacteria (LAB) and other organisms.