A mutant (DNA GC 70%) of Pseudonionas testosteroni (DNA GC 60%) has been shown t o produce new strains of P. testosteroni having the same GC content as the parent strain 78 (60 moles yo). Analyses of the protein and analogous enzymes by electrophoresis reveal an acidic alteration in the charge of all soluble proteins. Evaluation of the organisms in the Bacillus and Paracolon systems clearly demonstrates that total protein charge alterations from acidic to basic or basic to acidic are common features associated with alterations of DNA base compositions. The reported DNA content changes of mutants appears to follow a rule of single or multiple 25% substitution of parental adenine-thymine content to guanine-cytosine content in the mutant produced. The accuracy of this prediction of such a percentage change in mutant DNA content is presented.
The common features of extent of GC alteration and protein charge changes observed in the Pseudomonas, Bacillus, and Paracolon systems suggest the possibility of a single mutational event producing an organism which is classified in a genus other then the parent.
Summaries of DNA guanine-cytosine (GC) contents (HILL 1966) of various genera and species of bacteria indicate that taxonomically closely related species have nearly identical GC contents. The extent of this relationship, based on the base sequence of the DNA, can be tested with hybridization of single strands of DNA from the two strains. Normally, mutations are not associated with a major change in guanine-cytosine content nor are the phenotypic changes such as to necessitate reclassification of the mutant into another genus. mutation which is said to give rise to an organism classified in another genus would be designated an intergeneric event. Present concepts cannot accommodate such events. However, "mutations" resulting in extensive changes in GC content associated with major phenotypic changes have been reported. WEED (Duc-NGUYEN and WEED 1964, KELLY and WEED 1965, WEED 1963). has observed extensive DNA GC content changes in copper sulfate treated Bacillus subtilis 168. The gram positive bacillus is reported to have produced gram negative organisms with GC contents of 55 and 69-71 moles %. This organism, 4 G, with a GC content of 55 moles yo has been classified as an Erwinia-like organism. A small colony form detected as a peripheral spreading of the 4 G colony was cloned and found to have a DNA GC content of 69-71 moles yo. This organism designated 4G-SC was classified as Cornybacteriurn aqunticunt (Duc-NGUYEN and WEED 1964). GAUSE et al. (1967) have reported finding similar mutants in their studies with Paracolon and Staphylococcus aureus. Ultraviolet irradiation was reported to produce a mutant (mutant 1975) having a DNA GC content of 70.5 moles yo.