THE ROLE OF THE AUU INITIATION CODON IN THE NEGATIVE FEEDBACK-REGULATION OF THE GENE FOR TRANSLATION INITIATION-FACTOR IF3 IN ESCHERICHIA-COLI

The expression of the infC gene encoding translation initiation factor IF3 is negatively autoregulated at the level of translation, i.e. the expression of the gene is derepressed in a mutant infC background whe re the IF3 activity is lower than that of the wild type. The special i nitiation codon of infC, AUU, has previously been shown to be essentia l for derepression in vivo. In the present work, we provide evidence t hat the AUU initiation codon causes derepression by itself, because if the initiation codon of the thrS gene, encoding threonyl-tRNA synthet ase, is changed from AUG to AUU, its expression is also derepressed in an infC mutant background. The same result was obtained with the rpsO gene encoding ribosomal protein S15. We also show that derepression o f infC, thrS, and rpsO is obtained with other 'abnormal' initiation co dons such as AUA, AUC, and CUG which initiate with the same low effici ency as AUU, and also with ACG which initiates with an even lower effi ciency. Under conditions of IF3 excess, the expression of infC is repr essed in the presence of the AUU or other 'abnormal' initiation codons . Under the same conditions and with the same set of 'abnormal' initia tion codons, the repression of thrS and rpsO expression is weaker. Thi s result suggests that the infC message has specific features that ren der its expression particularly sensitive to excess of IF3. We also st udied another peculiarity of the infC message, namely the role of a GC -rich sequence located immediately downstream of the initiation codon and conserved through evolution. This sequence was proposed to interac t with a conserved region in 16S RNA and enhance translation initiatio n. Unexpectedly, mutating this GC-rich sequence increases infC express ion, indicating that this sequence has no enhancing role. Chemical and enzymatic probing of infC RNA synthesized in vitro indicates that thi s GC-rich sequence might pair with another region of the mRNA. On the basis of our in vivo results we propose, as suspected from earlier in vitro results, that IF3 regulates the expression of its own gene by us ing its ability to differentiate between 'normal' and 'abnormal' initi ation codons.