EVIDENCE OF SELECTION FOR PROTEIN INTRONS IN THE RECAS OF PATHOGENIC MYCOBACTERIA

Protein introns are recently discovered genetic elements whose interve ning sequences are removed from a precursor protein by an unusual prot ein splicing reaction. This involves the excision of a central spacer molecule, the protein intron, and the religation of the amino- and car boxy-terminal fragments of the precursor. The recA gene of Mycobacteri um tuberculosis contains one such element and we now show that the oth er major mycobacterial pathogen, Mycobacterium leprae, also possesses a protein intron in its recA, although other mycobacterial recA genes do not. However, these two protein introns are different in size, sequ ence and location of insertion of their coding sequences into the recA s of M.tuberculosis and M.leprae, indicating that acquisition of the p rotein introns has occurred independently in the two species, and thus suggesting that there has been selection for splicing in the maturati on of RecA in the pathogenic mycobacteria. The M.leprae protein intron provides an example of conditional protein splicing, splicing occurri ng in M.leprae itself but not when expressed in Escherichia coli, unli ke most previously described protein introns. These observations sugge st that protein introns may perform a function for their host, rather than being just selfish elements.