Mutations to spectinomycin resistance which alleviate the restriction of an amber suppressor by streptomycin resistance.

Unlike mutations to streptomycin resistance, which restrict the function of an SU II amber suppressor, mutations to spectinomycin resistance have no restrictive effect. Furthermore, mutations to spectinomycin resistance counteract the restrictive action of streptomycin-resistant mutations when combined in a single strain. The mutations to drug resistance, singly or in combination, do not themselves lead to any detectable suppressor activity. The effects on SU II are interpreted in terms of interactions between ribosomal elements.