HOMING EVENTS IN THE GYRA GENE OF SOME MYCOBACTERIA

The A subunit of DNA gyrase in Mycobacterium leprae, unlike its counte rpart in Mycobacterium tuberculosis, is produced by protein splicing a s its gene, gyrA, harbors a 1260-bp in-frame insertion encoding an int ein, a putative homing endonuclease, Analysis of the gyrA locus from d ifferent mycobacterial species revealed the presence of inteins in Myc obacterium flavescens, Mycobacterium gordonae, and Mycobacterium kansa sii but not in 10 other pathogenic or saprophytic mycobacteria, In all four cases where intein coding sequences were found, they were locali zed in the same position in gyrA, immediately downstream of the codon for the key active-site residue Tyr-130, The intein products were simi lar, but not identical, in sequence and the splice junctions displayed all the features found in other polypeptides known to be produced by protein splicing from a precursor protein, Paired motifs, found in hom ing endonucleases encoded by some group I RNA introns, and inteins sho wing endonuclease activity, were present in the gyrA inteins as were o ther intein-specific signatures, Some strains of M. flavescens, M. gor donae, and M. kansasii were shown by PCR analysis to have inteinless g yrA genes, in contrast to the situation in M. leprae where all the iso lates possessed insertions in gyrA. Sequencing of the corresponding re gions revealed that, although the GyrA protein sequence was conserved, the nucleotide sequences differed in gyrA genes with and without inte ins, suggesting that the homing endonuclease displays sequence specifi city.