The role of the AUU initiation codon in the negative feedback regulation of the gene for translation initiation factor IF3 in Escherichia coli

Abstract

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 where the IF3 activity is lower than that of the wild type. The special initiation codon of infC, AUU, has previously been shown to be essential for derepression in vivo. In the present work, we provide evidence that the AUU initiation codon causes derepression by itself, because if the initiation codon of the thrS gene, encoding threonyl-tRNA synthetase, 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 of infC, thrS, and rpsO is obtained with other 'abnormal' initiation codons such as AUA, AUC, and CUG which initiate with the same low efficiency as AUU, and also with ACG which initiates with an even lower efficiency. Under conditions of IF3 excess, the expression of infC is repressed in the presence of the AUU or other 'abnormal' initiation codons. Under the same conditions and with the same set of 'abnormal' initiation codons, the repression of thrS and rpsO expression is weaker. This result suggests that the infC message has specific features that render its expression particularly sensitive to excess of IF3. We also studied 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 interact with a conserved region in 16S RNA and enhance translation initiation. Unexpectedly, mutating this GC-rich sequence increases infC expression, indicating that this sequence has no enhancing role. Chemical and enzymatic probing of infC RNA synthesized in vitro indicates that this 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 using its ability to differentiate between 'normal' and 'abnormal' initiation codons.

The Translational Fidelity Function of IF3 During Transition from the 30 S Initiation Complex to the 70 S Initiation Complex

“…The presence of IF3 introduces a translational bias which reduces or prevents translation starting from noncanonical initiation complexes, pseudo-initiation complexes, or complexes containing leaderless mRNA1,2, preventing thereby formation of an "incorrect" 30SIC. This "fidelity function" of IF3 has also been described in vivo3, 10 and provides the molecular basis for the translational auto-regulation of infC expression by IF311-13. fMet-tRNA fMet binding to the 30SIC is reversible, but becomes essentially irreversible following 30SIC association with 50S subunits and formation of the 70S initiation complex (70SIC).…”

Functional importance of RNA interactions in selection of translation initiation codons

“…Start-site selection of bacterial mRNAs includes a second type of RNA base pairing between the start codon and the anticodon loop of the initiator fMet-tRNA fMet , the fidelity of which is controlled by the translation initiation factor IF3 (Sacerdot et al, 1996;Sussman et al, 1996). The specificity of this interaction is also governed by initiation factor IF2 which recognizes the initiator tRNA and probably helps to correctly position it near the ribosomal decoding centre (Voorma, 1996).…”

In vivo involvement of mutated initiation factor IF1 in gene expression control at the translational level

“…Even though binding of IF1 to the ribosomal A-site could suggest that IF1 takes part in an initiation fidelity function [20,21], this hypothesis has not been tested so far (latest review [1]). In this study, we have investigated the relationship between IF1 and the nature of the +2 codon immediately following the initiation codon, thus representing the ribosomal Asite.…”