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Journal of Bacteriology, July 2008, p. 4791-4797, Vol. 190, No. 14
0021-9193/08/$08.00+0     doi:10.1128/JB.00290-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Conserved Residues Asp16 and Pro24 of TnaC-tRNAPro Participate in Tryptophan Induction of tna Operon Expression{triangledown}

Luis R. Cruz-Vera{dagger} and Charles Yanofsky*

Department of Biological Sciences, Stanford University, Stanford, California 94305

Received 26 February 2008/ Accepted 5 April 2008

In Escherichia coli, interactions between the nascent TnaC-tRNAPro peptidyl-tRNA and the translating ribosome create a tryptophan binding site in the ribosome where bound tryptophan inhibits TnaC-tRNAPro cleavage. This inhibition delays ribosome release, thereby inhibiting Rho factor binding and action, resulting in increased tna operon transcription. Replacing Trp12 of TnaC with any other amino acid residue was previously shown to prevent tryptophan binding and induction of tna operon expression. Genome-wide comparisons of TnaC amino acid sequences identify Asp16 and Pro24, as well as Trp12, as highly conserved TnaC residues. Replacing these residues with other residues was previously shown to influence tryptophan induction of tna operon expression. In this study, in vitro analyses were performed to examine the potential roles of Asp16 and Pro24 in tna operon induction. Replacing Asp16 or Pro24 of TnaC of E. coli with other amino acids established that these residues are essential for free tryptophan binding and inhibition of TnaC-tRNAPro cleavage at the peptidyl transferase center. Asp16 and Pro24 are in fact located in spatial positions corresponding to critical residues of AAP, another ribosome regulatory peptide. Sparsomycin-methylation protection studies further suggested that segments of 23S RNA were arranged differently in ribosomes bearing TnaCs with either the Asp16Ala or the Pro24Ala change. Thus, features of the amino acid sequence of TnaC of the nascent TnaC-tRNAPro peptidyl-tRNA, in addition to the presence of Trp12, are necessary for the nascent peptide to create a tryptophan binding/inhibition site in the translating ribosome.

* Corresponding author. Mailing address: Department of Biological Sciences, Stanford University, Stanford, CA 94305. Phone: (650) 725-1835. Fax: (650) 725-8221. E-mail: yanofsky{at}stanford.edu

{triangledown} Published ahead of print on 18 April 2008.

{dagger} Present address: Department of Biological Sciences, University of Alabama in Huntsville, Huntsville, AL 35899.

Journal of Bacteriology, July 2008, p. 4791-4797, Vol. 190, No. 14
0021-9193/08/$08.00+0     doi:10.1128/JB.00290-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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