The pathogen Mycobacterium marinum, a faster growing close relative of Mycobacterium tuberculosis, has a single rRNA operon per genome.

Although Mycobacterium marinum and Mycobacterium tuberculosis are very closely related they differ significantly in their growth rates. The Type strain of M. marinum and one clinical isolate were investigated and, like M. tuberculosis, were found to have a single rRNA (rrn) operon per genome located downstream from murA gene and controlled by two promoters. No sequence differences were found that account for the difference in the growth rates of the two species. We infer that M. tuberculosis has the capacity to synthesize rRNA much faster than it actually does; and propose that the high number of insertion sequences in this species attenuate growth rate to lower values.