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Cleared lysates of a proteolytic (Prt+ ) strain and a naturally occurring non-proteolytic (Prt− ) variant ofStreptococcus cremoris Wg2 contain equal amounts of covalently closed circular plasmid DNA. An analysis of this plasmid DNA by agarose gel electrophoresis revealed the presence of at least five different plasmid species in the Prt+ strain and only three plasmid species in the Prt− variant. Curing studies with acriflavine indicated that a 16-megadalton plasmid determined proteolytic activity in the Prt+ strain. In energy-limited chemostats inoculated with both strains it was observed that the Prt+ strain was replaced by the Prt− variant. This effect was most apparent when the pH of the culture was fixed at a value above 6.3. No selection for the Prt− variant was observed at pH 5.9. Since the two types of organisms contain equal amounts of plasmid DNA, it was concluded that the energy gain of the Prt− variants at pH values above 6.0 probably has to be found in protein synthesis rather than in plasmid DNA synthesis.

Plasmid DNA in Streptococcus cremoris Wg2: Influence of pH on Selection in Chemostats of a Variant Lacking a Protease Plasmid