Conformational changes and reassociation properties of small subunits of Escherichia coli ribosomes

Studies on the reassociation of ribosomal subunits from Escherichia coli were carried out using an ultracentrifuge equipped with a UV absorption scanner, and the conformational changes of ribosomal subunits were followed by circular dichroism measurements. Reassociation between the fractionated 30S and 50 S subunits was promoted by the addition of uncharged tRNA at 4 degrees. Furthermore, a sharp increase in the amount of reassociated products was observed on treatment of the sample solution at temperatures above 25 degrees. Treatment at higher temperature only affected 30 S ribosomal slbunits. From CD measurements of Mg2+ concentration dependence for both ribosomal subunits, a significant decrease in molar ellipticity [theta] at the 264 nm positive peak and a slight change of [theta] at the 222 nm band were observed for both subunits in dissociation buffer. Samples of 30 S subunits pretreated at 37 degrees for 40 min showed a small CD change at the 222 nm band. The same samples showed a slight increase in S value and enhanced ability to form 70 S particles with 50 S subunits. On the other hand, we detected no significant changes in CD spectra, S value or the ability to form 70 S particles with 50 S ribosomal samples subjected to same heat treatments.