A Fragment of21ORFs Around the Direct Repeat (DR) Region ofMycobacterium tuberculosisis Absent From the Other Sequenced Mycobacterial Genomes: Implications for the Evolution of the DR Region

摘要

The direct repeat (DR) region is a singular locus of theMycobacterium tuberculosiscomplex genome. This region consists of 36 bp repetitive sequences separated bynon-repetitive unique spacer sequences. Around this region there are several genescoding for proteins of unknown function. To determine whether theM. smegmatis, M.avium, M. marinumandM. lepraegenomes contain sequences and ORFs similar tothose of the DR locus of theM. tuberculosiscomplex, we analysed the correspondingregions in these species. As a first step, some conserved genes that flank the DR genes[Rv2785c (rpsO), Rv2786c (ribF), Rv2790c (ltp1 ), Rv2793c (truB), Rv2800, Rv2825,Rv2828, Rv2831 (echA16 ), Rv2838 (rbfA)andRv2845 (proS )] were used as markersto locate the corresponding orthologues inM. smegmatis, M. avium, M. marinumandM. leprae in silico. Most of theseM. tuberculosismarker genes have highly similarorthologues located in the same order and orientation in the other mycobacteria. Incontrast, no orthologues were found for ORFsRv2801–Rv2824, suggesting that thesegenes are unique toM. tuberculosiswithin the genusMycobacterium.We observed thatinM. smegmatisandM. avium, Rv2800andRv2825are adjacent.This observation was experimentally confirmed by PCR. In conclusion, as the DR locus and the ORFsaround it are absent inM. smegmatisandM. aviumand, as it is possible that thesespecies are older thanM. tuberculosis, we postulated that the DR locus was acquiredby theM. tuberculosiscomplex species or by an ancestor bacterium.