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The Bacillus subtilis tyrZ Gene Encodes a Highly Selective Tyrosyl-tRNA Synthetase and Is Regulated by a MarR Regulator and T Box Riboswitch

Williams-Wagner, Rebecca N., Grundy, Frank J., Raina, Medha, Ibba, Michael, Henkin, Tina M.
Journal of bacteriology 2015 v.197 no.9 pp. 1624-1631
transcription factors, aminoacylation, biofilm, transfer RNA, mutation, amino acids, operon, Bacillus subtilis, toxicity
Misincorporation of d -tyrosine (d -Tyr) into cellular proteins due to mischarging of tRNA ᵀʸʳ with d -Tyr by tyrosyl-tRNA synthetase inhibits growth and biofilm formation of Bacillus subtilis . Furthermore, many B. subtilis strains lack a functional gene encoding d -aminoacyl-tRNA deacylase, which prevents misincorporation of d -Tyr in most organisms. B. subtilis has two genes that encode tyrosyl-tRNA synthetase: tyrS is expressed under normal growth conditions, and tyrZ is known to be expressed only when tyrS is inactivated by mutation. We hypothesized that tyrZ encodes an alternate tyrosyl-tRNA synthetase, expression of which allows the cell to grow when d -Tyr is present. We show that TyrZ is more selective for l -Tyr over d -Tyr than is TyrS; however, TyrZ is less efficient overall. We also show that expression of tyrZ is required for growth and biofilm formation in the presence of d -Tyr. Both tyrS and tyrZ are preceded by a T box riboswitch, but tyrZ is found in an operon with ywaE , which is predicted to encode a MarR family transcriptional regulator. Expression of tyrZ is repressed by YwaE and also is regulated at the level of transcription attenuation by the T box riboswitch. We conclude that expression of tyrZ may allow growth when excess d -Tyr is present. IMPORTANCE Accurate protein synthesis requires correct aminoacylation of each tRNA with the cognate amino acid and discrimination against related compounds. Bacillus subtilis produces d -Tyr, an analog of l -Tyr that is toxic when incorporated into protein, during stationary phase. Most organisms utilize a d -aminoacyl-tRNA deacylase to prevent misincorporation of d -Tyr. This work demonstrates that the increased selectivity of the TyrZ form of tyrosyl-tRNA synthetase may provide a mechanism by which B. subtilis prevents misincorporation of d -Tyr in the absence of a functional d -aminoacyl-tRNA deacylase gene.