Ribosomal protein L4 from Escherichia coli utilizes nonidentical determinants for its structural and regulatory functions.

Escherichia coli ribosomal protein L4 has two functions: it is a structural component of the 50S ribosomal sub-unit and it is a repressor of both transcription and translation of its own transcription unit, the 11-gene S10 operon. Genetic and biochemical studies have suggested that L4 can interact with 23S rRNA as well as with both RNA interactions. However, no significant similarities between its two RNA targets can be found at the primary or secondary structure level. To test if identical determinants of L4 are involved in both ribosome assembly and autogenous control, we have isolated L4 mutants defective in either of these functions and asked if a mutant protein divested of one function is also deficient in the other. Several mutations eliminated autogenous control, but still allowed assembly of the mutant L4 protein into functional ribosomes. Conversely, several mutant L4 proteins that could not be detected in 50S subunits nevertheless could regulate expression of the S10 operon. These results indicate that the L4 determinants required for autogenous regulation and ribosome incorporation are not congruent.