IDENTIFICATION OF NOVEL INTERGENIC REPETITIVE UNITS IN A MYCOBACTERIAL 2-COMPONENT SYSTEM OPERON

Mycobacterial interspersed repetitive units (MIRUs), a novel class of repeated sequences, were identified within the intercistronic region o f an operon coding for a mycobacterial two-component system, named sen X3-regX3. Southern blot analysis and homology searches revealed the pr esence of several homologous sequences in intergenic regions dispersed throughout the genomes of Mycobacterium bovis BCG, Mycobacterium tube rculosis and Mycobacterium leprae, These could be grouped into three m ajor families, containing elements of 77-101 bp, 46-53 bp and 58-101 b p, Based on the available mycobacterial sequences, the total number of MIRUs is estimated to be about 40-50 per genome, Similar to previousl y identified small repetitive sequences, the MIRUs of the two-componen t operon are transcribed on a polycistronic mRNA, Unlike previously id entified small repetitive sequences, however, MIRUs do not contain dya d symmetries, comprise small open reading frames (ORFs) whose extremit ies overlap those of the contiguous ORFs and are oriented in the same translational direction as those of the adjacent genes, Analyses of th e sequences at the insertion sites suggest that MIRUs disseminate by t ransposition into DTGA sites involved in translational coupling in pol ycistronic operons.