Heterogeneity of deoxyribonucleic acid molecules isolated from Mycobacterium smegmatis

Bulk chromosomal deoxyribonucleic acids (DNAs) of M. smegmatis strains 607⁺ (wild type) and 607-1 (Str(r)) and orange-red pigmented variants (OR) were separated into two distinct bands (types 1 and 2) by cesium chloride density gradient centrifugation. Thermal denaturation analyses showed that type 1 and 2 DNA fragments of these strains possessed quanine plus cytosine contents averaging 69.2% and 60.8%, respectively. Type 1 and 2 DNAs from all strains tested were recovered in relatively equal quantities upon isolation and were found to have similar molecular weights (3.0x10⁷). Spectrophotometric assay of DNA reassociation showed that homology between any type 1 and 2 DNA fragments was always very low (29 to 33%), even within the same strain. Homologies among type 1 DNAs isolated from any strain were always high (92 to 98%), whereas homologies between type 2 DNA isolated from OR strains and that from their parental strain 607-1 were lower (51 to 55%). Transformation experiments revealed that methionine, leucine, folic acid, and streptomycin markers were found exclusively in type 1 DNA fragments. In addition to the two types of chromosomal DNA, plasmid DNA possessing a molecular weight of about 4x10⁶ was found in strain 607-1.