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The activation of Escherichia coli K12 3300 β-galactosidase by Na+ and Mg2+ ions appears to involve, apart from effects on the catalytic rate, a complex pattern of interaction between the substrate and the two cations. The time course of activation by Mg2+ can be followed directly by rapid mixing methods; m the presence of Na+ the kinetics of this process is complex, due to the presence of a monomolecular step which becomes rate limiting at high reagent concentrations. The effects produced by Na+, on the other hand, occur rapidly in comparison with the highest attainable rates of catalysis. The activity of defective enzymes from lac- point mutants is also Na+- and Mg2+-dependent. [ABSTRACT FROM AUTHOR]

We constructed two aptamers, each of which contains a 7-nt-long loop complementary to the anticodon loop of a suppressor tRNA. One of these aptamers can form a stable bimolecular complex with the suppressor tRNA in vitro and protects the 7 nt in the suppressor's anticodon loop from RNase S1. An Escherichia coli strain, carrying an amber mutation in the lac Z gene, produces beta-galactosidase only if the suppressor is present; the aptamer's coexpression in the cell inhibits the activity of the suppressor tRNA. Moreover, in E. coli extract, the aptamer partially inhibits the read-through of the stop codon on the part of the suppressor tRNA. These results point to a novel strategy that need not be limited to the suppressor tRNA. By constructing appropriate inducible aptamers, it may well be possible to effectively control translation in vivo.