Evidence of selection for protein introns in the recAs of pathogenic mycobacteria

Protein introns are recently discovered genetic elements whose intervening sequences are removed from a precursor protein by an unusual protein splicing reaction. This involves the excision of a central spacer molecule, the protein intron, and the religation of the amino- and carboxy-terminal fragments of the precursor. The recA gene of Mycobacterium tuberculosis contains one such element and we now show that the other 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, sequence and location of insertion of their coding sequences into the recAs of M. tuberculosis and M. leprae, indicating that acquisition of the protein introns has occurred independently in the two species, and thus suggesting that there has been selection for splicing in the maturation of RecA in the pathogenic mycobacteria. The M. leprae protein intron provides an example of conditional protein splicing, splicing occurring in M. leprae itself but not when expressed in Escherichia coli, unlike most previously described protein introns. These observations suggest that protein introns may perform a function for their host, rather than being just selfish elements.