One of the primary rRNA binding proteins, this protein initially binds near the 5'-end of the 23S rRNA. It is important during the early stages of 50S assembly. It makes multiple contacts with different domains of the 23S rRNA in the assembled 50S subunit and ribosome (By similarity). This protein only weakly controls expression of the E.coli S10 operon. It is incorporated into E.coli ribosomes, however it is not as firmly associated as the endogenous protein. Forms part of the polypeptide exit tunnel. Part of the 50S ribosomal subunit. The structure indicates that the N-terminal domain may bind to the 23S rRNA, while the C-terminal domain may bind to the mRNA, which could then be implicated in transcriptional and translational control of the S10 operon. However, it is not known if the S10 operon is controlled in this fashion in this organism. A protein containing 11 mutations, chosen as amino acids which probably interact with nucleic acids (K140A, E141Q, Q144K, Q147A, S149E, K151delta, K152delta, I155M, P158G, W159E and K160L), gained the ability to regulate the E.coli S10 operon, suggesting it is now able to mediate contacts of L4 with the S10 mRNA leader in E.coli. This mutant protein did not associate any better with E.coli ribosomes however, indicating a separation of residues interacting with rRNA and mRNA. Belongs to the universal ribosomal protein uL4 family.