Summary
A possible quantitative system for the interconversion of ochre and amber suppressors was studied in Escherichia coli WU36-10, a strain in which a leucine requirement is suppressed by amber suppressors and a tyrosine requirement is suppressed by ochre suppressors. The conversion of am Sup-2+ to oc Sup-2+ occurred at rates similar to those for the de novo induction of such suppressors, both spontaneously and after ultraviolet or gamma irradiation. Both induction and conversion of suppressors showed the phenomenon of “mutation frequency decline” after ultraviolet light. Conversions in the opposite direction from oc Sup-2+ to am Sup-2+ were, however, not detected in unmutagenised populations of oc Sup-2+ strains derived either by conversion from an am Sup-2+ strain or de novo from the parental WU36-10, nor were they detected after treatment with ultraviolet light, gamma radiation or 2-aminopurine. If the conversion of oc Sup-2+ to am Sup-2+ occurs at all, it is at a rate very considerably lower than that for the conversion of am Sup-2+ to oc Sup-2+. Some Tyr+ oc Sup-2+ mutants demonstrated mutation rates c. 100 times greater than those of WU36-10 for mutation to Leu+ spontaneously and after ultraviolet or gamma radiation. Possible explanations of this are discussed.
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Bridges, B.A., Dennis, R.E. & Munson, R.J. Mutagenesis in Escherichia coli V. Attempted interconversion of ochre and amber suppressors and mutational instability due to an ochre suppressor. Molec. Gen. Genet. 107, 351–360 (1970). https://doi.org/10.1007/BF00441196
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DOI: https://doi.org/10.1007/BF00441196