Streptomycin Inhibition of Protein Synthesis in Peas reversed by Divalent Cations

Abstract

STREPTOMYCIN inhibits the growth of higher plants^1,2 and causes bleaching of chloroplasts^3,4. Because these effects largely disappear in the presence of divalent cations, notably Mg⁺⁺, Mn⁺⁺, Ca⁺⁺, it has been suggested^2,5 that the phytotoxic action of streptomycin is related to competition with divalent cations for certain intracellular binding sites. I have shown that the inhibition of protein synthesis caused by streptomycin in excised plant tissues⁶ is overcome by these same divalent cations, but that this reversal is the consequence of an inhibition of the uptake of streptomycin. Incorporation of leucine into protein was determined as described earlier⁶, using apical third internode segments of etiolated pea epicotyls. The excised segments were incubated in the appropriate sterile medium (buffered in 5 mM potassium maleate pH. 5.5 and containing 15 mg/l. of penicillin G) for 1 h, before addition of L-leucine-l-¹⁴C (New England Nuclear Corp., 31.0 mCi/mM) for a further 2 h.