The alr-groEL1 operon in Mycobacterium tuberculosis: an interplay of multiple regulatory elements

Threonylcarbamoyladenosine is a universally conserved essential modification of tRNA that ensures translational fidelity in cellular milieu. TsaD, TsaB and TsaE are identified as tRNA-A₃₇-threonylcarbamoyl (t⁶A)-transferase enzymes that have been reconstituted in vitro, in few bacteria recently. However, transcriptional organization and regulation of these genes are not known in any of these organisms. This study describes the intricate architecture of a complex multicistronic alr-groEL1 operon, harboring essential genes, namely tsaD, tsaB, tsaE, groES, groEL1, and alr (required for cell wall synthesis), and rimI encoding an N-α- acetyltransferase in Mycobacterium tuberculosis. Using northern blotting, RT-PCR and in vivo fluorescence assays, genes alr to groEL1 were found to constitute an ~6.3 kb heptacistronic operon with multiple internal promoters and an I-shaped intrinsic hairpin-like cis-regulatory element. A strong promoter PtsaD within the coding sequence of rimI gene is identified in M. tuberculosis, in addition. The study further proposes an amendment in the known bicistronic groESL1 operon annotation by providing evidence that groESL1 is co-transcribed as sub-operon of alr-groEL1 operon. The architecture of alr-groEL1 operon, conservation of the genetic context and a mosaic transcriptional profile displayed under various stress conditions convincingly suggest the involvement of this operon in stress adaptation in M. tuberculosis.