The maintenance and genetic stability of the vector plasmids pBR322 and pBR325 in two genetically different Escherichia coli hosts were studied during chemostat cultivation with glucose and ammonium chloride limitation and at two different dilution rates. The plasmid pBR322 was stably maintained under all growth conditions tested. However pBR325 segregated from both hosts preferentially during glucose limitation and at low dilution rate. In addition to this general segregation process a separate loss of tetracycline resistance was observed. The remaining plasmid conferred resistance to ampicillin and chloramphenicol only, without any remarkable alteration of its molecular weight. Cultivation conditions in the chemostat were found that allowed the stable genetic inheritance of both plasmids in the hosts studied.

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A simple method has been worked out for measuring the biologically effective dose (BED) of solar radiation. The method uses phage T7 as a biosensor and it includes field measurements of global and direct UV radiation from the sun in the air; it has been applied to underwater measurements as well. Results of field measurements are presented, with discussion of the angle-dependent sensitivity of the biosensor. A model of spectral irradiance based on the measured values is presented. Relevance of the HT7 unit-derived earlier by us from T7 phage inactivation upon UV radiation-as a measure of the BED is also discussed.