Gene replacement and expression of foreign DNA in mycobacteria.

A system that permits molecular genetic manipulation of mycobacteria was developed on the basis of the yeast paradigm of gene replacement by homologous recombination. A shuttle vector that can replicate autonomously at a high copy number in Escherichia coli but must integrate into homologous DNA for survival in Mycobacterium smegmatis was constructed. The vector contains a ColE1 origin of replication, antibiotic resistance markers for ampicillin and kanamycin, a nutritional marker (pyrF) that allows both positive and negative selection in E. coli and M. smegmatis, and unique restriction sites that permit insertion of foreign DNA. Transformation of mycobacteria with this vector results in integration of its DNA into the genomic pyrF locus by either a single or a double homologous recombination event. With this system, the 65-kilodalton Mycobacterium leprae stress protein antigen was inserted into the M. smegmatis genome and expressed. This gene replacement technology, together with a uniquely useful pyrF marker, should be valuable for investigating mycobacterial pathobiology, for the development of candidate mycobacterial vaccine vehicles, and as a model for the development of molecular genetic systems in other pathogenic microorganisms.