Tagging ribosomal protein S7 allows rapid identification of mutants defective in assembly and function of 30 S subunits

Ribosomal protein S7 nucleates folding of the 16 S rRNA 3' major domain, wh ich ultimately forms the head of the 30 S ribosomal subunit. Recent crystal structures indicate that 57 lies on the interface side of the 30 S subunit , near the tRNA binding sites of the ribosome. To map the functional surfac e of S7, we have tagged the protein with a Protein Kinase A recognition sit e and engineered alanine substitutions that target each exposed, conserved residue. We have also deleted conserved features of 57, using its structure to guide our design. By radiolabeling the tag sequence using Protein Kinas e A, we are able to track the partitioning of each mutant protein into 30 S , 70 S, and polyribosome fractions in vivo. Overexpression of 57 confers a growth defect, and we observe a striking correlation between this phenotype and proficiency in 30 S subunit assembly among our collection of mutants. We find that the side chain of K35 is required for efficient assembly of S7 into 30 S subunits in vivo, whereas those of at least 17 other conserved e xposed residues are not required. In addition, an 57 derivative lacking the N-terminal 17 residues causes ribosomes to accumulate on mRNA to abnormall y high levels, indicating that our approach can yield interesting mutant ri bosomes. (C) 2000 Academic Press.