More recently, a set of non-synonymous mutations in the genes
that encode RNA polymerase subunits RpoA and RpoC were
implicated in the compensatory evolution of RIF resistance.72-74
These gene sequences are generally highly conserved in M.tb, so
non-synonymous mutations are likely to represent recently
acquired adaptive mutations and not natural polymorphisms.72
These putative compensatory mutations, which appear to be
preferably located in the rpoA–rpoC interaction region of the rpoC
gene, have been associated with increased in vitro fitness73,75,76
and are often present in MDR-TB strains.72-74 The acquisition of
rpoA-rpoC mutations generally occurs in strains that are already
RIF-resistant due to alterations in the rpoB gene77, in a process that
seems to be facilitated by the continued exposure to drugs.76 This
compensatory evolution leads to improved transmissibility as
evidenced by the clonal expansion of M.tb strains harbouring such
combinations of resistance and compensatory mutations.72,73
Interestingly, these changes are frequently present in strains that
harbour rpoB resistance mutations associated with the lowest in
vitro fitness deficits.72,74 For example, they are often co-present
with rpoB mutation S531L,72 simultaneously one of the most
frequent mutations in RIF-resistant clinical isolates and one of the
least costly.