Identification of a region of Escherichia coli ribosomal protein L2 required for the assembly of L16 into the 50 S ribosomal subunit

In vitro mutagenesis of rplB was used to generate changes in a conserved region of Escherichia coli ribosomal protein L2 between Gly²²¹ and His²³¹. Mutants were selected by temperature sensitivity using an inducible expression system. A mutant L2 protein with the deletion of Thr²²² to Asp²²⁸ was readily distinguishable from wild-type L2 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and ribosomes from the strain overexpressing this mutant protein were characterized by sucrose density gradient centrifugation and protein composition. In addition to 30 S and 50 S ribosomal subunits, cell lysates contained a new component that sedimented at 40 S in 1 mM Mg²⁺ and at 48 S in 10 mM Mg²⁺. These particles contained mutant L2 protein exclusively, completely lacked L16, and had reduced amounts of L28, L33, and L34. They did not reassociate with 30 S ribosomal subunits and were inactive in polyphenylalanine synthesis. Other mutants in the same conserved region, including the substitution of His²²⁹ by Gln²²⁹, produced similar aberrant 50 S particles that sedimented at 40 S and failed to associate with 30 S subunits.