Antisuppression by a mutation in rpsM(S13) giving a shortened ribosomal protein S13

The phenotype associated with an rpsM(S13) mutation, originally isolated in Escherichia coli in a selection for pseudoreversion of streptomycin dependence, was studied in strains lacking the original mutations for antibiotic dependence. The rpsM mutation gives a decreased translational step time and a reduced growth rate. It functions as a strong antisuppressor 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 reduced sedimentation coefficient but is able to form apparently normal 70S ribosomes as judged by ultracentrifugational analysis. Cloning and sequencing show that the rpsM mutation is a CAG to TAG alteration at codon position 100, giving an S13 protein which is shortened by 19 amino 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 should be affected.