부산대학교 도서관

Ribosomal protein S1, the product of the essential rpsA gene, consists of six imperfect repeats of the same motif. Besides playing a critical role in translation initiation on most mRNAs, SI also specifically autoregulates the translation of its own messenger, ssyF29 is a viable rpsA allele that carries an IS10R insertion within the coding sequence, resulting in a protein lacking the last motif (S[1.sup.[DELTA]C]). The growth ofssyF29 cells is slower than that of wild-type cells. Moreover, translation of a reporter rpsA-lacZ fusion is specifically stimulated, suggesting that the last motif is necessary for autoregulation. However, in ssyF29 cells the rpsA mRNA is also strongly destabilized; this destabilization, by causing S[1.sup.[DELTA]C] shortage, might also explain the observed slow-growth and autoregulation defect. To fix this ambiguity, we have introduced an early stop codon in the rpsA chromosomal gene, resulting in the synthesis of the S[1.sup.[DELTA]C] protein without an IS10R insertion (rps[A.sup.[DELTA]c] allele), rpsA.sup.[DELTA]c] cells grow much faster than their ssyF29 counterparts; moreover, in these cells S1 autoregulation and mRNA stability are normal. In vitro, the S[1.sup.[DELTA]c] protein binds mRNAs (including its own) almost as avidly as wild-type S1. These results demonstrate that the last S1 motif is dispensable for translation and autoregulation: the defects seen with ssyF29 cells reflect an IS10R-mediated destabilization of the rpsA mRNA, probably due to facilitated exonucleolytic degradation.