Role of Protein L25 and Its Contact with Protein L16 in Maintaining the Active State of Escherichia coli Ribosomes in vivo.

A ribosomal protein of the L25 family specifically binding to 5S rRNA is an evolutionary feature of bacteria. Structural studies showed that within the ribosome this protein contacts not only 5S rRNA, but also the C-terminal region of protein L16. Earlier we demonstrated that ribosomes from the ΔL25 strain of Escherichia coli have reduced functional activity. In the present work, it is established that the reason for this is a fraction of functionally inactive 50S ribosomal subunits. These subunits have a deficit of protein L16 and associate very weakly with 30S subunits. To study the role of the contact of these two proteins in the formation of the active ribosome, we created a number of E. coli strains containing protein L16 with changes in its C-terminal region. We found that some mutations (K133L or K127L/K133L) in this protein lead to a noticeable slowing of cell growth and decrease in the activity of their translational apparatus. As in the case of the ribosomes from the ΔL25 strain, the fraction of 50S subunits, which are deficient in protein L16, is present in the ribosomes of the mutant strains. All these data indicate that the contact with protein L25 is important for the retention of protein L16 within the E. coli ribosome in vivo. In the light of these findings, the role of the protein of the L25 family in maintaining the active state of the bacterial ribosome is discussed.