DNA methylation in leprosy-associated bacteria: Mycobacterium leprae and Corynebacterium tuberculostearicum

The DNAs of two kinds of microorganisms from human leprosy lesion, Mycobacterium leprae and Corynebacterium tuberculostearicum (also known as leprosy-derived Corynebacterium or LDC), have been analysed and compared with the genomes of reference bacteria of the CMN group (genera Corynebacterium, Mycobacterium and Nocardia). The guanine-plus-cytosine content (% GC) of DNA was determined by a double-labelling procedure, which is unaffected by the presence of modified and unusual bases (that alter both buoyant density and mid-melting-point determinations). Accordingly, the DNAs of seven LDC strains had GC values of 54–56 mol %, and that of armadillo-grown M. leprae a value of 54.8±0.9 mol %. Restriction patterns disclosed no methylated cytosine in the DNA sequences CCGG, GGCC, AGCT and GATC of either LDC or M. leprae DNA. N⁶-methyl adenine was present in the sequence GATC of all LDC strains, but was missing from the genomes of all others CMN organisms analysed, including M. leprae. By HPLC analysis of LDC-DNA hydrolysates, it was found that N⁶-methyladenine amounted to 1.8% of total DNA adenine, and was present exclusively within GATC sequences, which appeared all to be methylated. It is concluded that LDC represent a group of corynebacteria endowed with high genetic homogeneity and a unique restriction pattern, whereby their genome is easily distinguished from that of M. leprae, which has a similar base composition.Abbreviations %GC guanine-plus-cytosine content of DNA (mol/100 mol) - T_m midpoint of optical absorption (A_260nm) melting curve of DNA - MeA N⁶ methyl-adenine - MeC 5 methyl-cytosine - HPLC high performance liquid chromatographyEnzyme Code DNA poll deoxynucleotide triphosphate - DNA deoxynucleotidyltransferase (EC 2.7.7.7) - RNAase I ribonuclease A (EC 3.1.27.5)