ANTISUPPRESSION BY A MUTATION IN RPSM(S13) GIVING A SHORTENED RIBOSOMAL-PROTEIN S13

The phenotype associated with an rpsM(S13) mutation, originally isolat ed in Escherichia coli in a selection for pseudoreversion of streptomy cin dependence, was studied in strains lacking the original mutations for antibiotic dependence. The rpsM mutation gives a decreased transla tional step time and a reduced growth rate. It functions as a strong a ntisuppressor to both the serU(Su1) amber suppressor and the trpT(Su9) opal suppressor, whereas the tyrT(Su3) amber suppressor is much less affected. The small ribosomal subunit from the rpsM mutant shows a red uced sedimentation coefficient but is able to form apparently normal 7 0S ribosomes as judged by urtracentrifugational analysis. Cloning and sequencing show that the rpsM mutation is a CAG to TAG alteration at c odon position 100, giving an S13 protein which is shortened by 19 amin o acids at its C-terminal end. This implies that the C-terminal domain of the protein that is involved in binding to 16S ribosomal RNA shoul d be affected.