Loosening and unfolding of E. coli 50 S ribosomal subunits: dependence on magnesium content and temperature.

Reversible change of 50 S ribosomal subunits to 40 S particles takes place in cold buffered 0.5 M NH4Cl solutions either containing Mg++ (up to 0.1 M), or free from Mg++ and even supplemented with EDTA (1 mM). The 40 S particles were stable only within a definite temperature range. Heating of the samples caused completely irreversible unfolding of the 40 S particles. This "melting" appeared to be co-operative and took place within a very narrow range of temperature, which for samples containing Mg++ was a linear function of the log of Mg++ concentration. The results suggest that two types of bonds maintained the compact structure of the ribosomal subunits: ionic bonds involving Mg++ and heat-labile weak interactions between ribosomal components.