The tRNA species for redundant genetic codons NNU and NNC. A thought on the absence of phenylalanine tRNA with AAA anticodon in Escherichia coli.

N Gavini,L Pulakat

doi:10.1016/s0021-9258(18)45868-1

Abstract

The redundant genetic codons NNU and NNC (where N is A, T, G, or C) specify the same amino acid and are decoded by their cognate tRNAs, which contain either a guanosine or a modified base in the wobble position of the anticodons. Since tRNAs with an adenosine in the wobble position of the anticodon, which are complementary to the NNU codons, are not found naturally, we have generated a tRNA(Phe) with AAA anticodon and examined how an adenosine in the wobble position would affect its biological function in Escherichia coli. We found that the tRNA(Phe) with GAA anticodon (wild-type) repressed the expression of the pheA gene via tRNA(Phe)-mediated attenuation of transcription, whereas the tRNA(Phe) with AAA anticodon did not influence the expression of the pheA gene. Furthermore, elevated levels of tRNA(Phe)(AAA) did not support the growth of an E. coli strain carrying a temperature-sensitive mutation in the pheS gene at 42 degrees C. Since the presence of a multicopy plasmid carrying the gene that encodes tRNA(Phe)(GAA), a substrate for phenylalanyl tRNA synthetase, enables the E. coli strain carrying the pheS(Ts) mutation to grow at 42 degrees C, the above observation suggests that unlike tRNA(Phe)(GAA), tRNA(Phe)(AAA) is not a good substrate for phenylalanyl-tRNA synthetase. Therefore, we postulate that the presence of adenosine at the wobble position of anticodons was specifically eliminated and the tRNAs with guanosine or a modified base in the wobble position were selected to decode both NNU and NNC codons in E. coli.

Highlights

From the DeDartment of Molecular Biolom and Biochemistry, School of Biological Sciences,University of California, i Irvine, Califo'rnia 9271
Since tRNAs with an adenosine in the wobble position of the anticodon, which are complementary to the NNU codons, are not found naturally, we have generated a tRNAPh'with AAA anticodonand examined how an adenosine in the wobble position would affect its biological function in Escherichia coli
We found that the tRNAPh'with GAA anticodon repressed the expression of the pheA gene via tRNAPhe-mediatedattenuation of transcription, whereas the tRNAPhewith AAA anticodon did not influence the expression ofthe pheA gene

Summary

Narasaiah GaviniS and Lakshmidevi Pulakat

From the DeDartment of Molecular Biolom and Biochemistry, School of Biological Sciences,University of California, i Irvine, Califo'rnia 9271. In thecase of tRNAPhe of E. coli in particular, it has been shown that a suppressor tRNA with anticodon CUA can insert phenylalanineinto the amber position, suggesting that a tRNA with the anticodon CUA which has the first two bases different from the GAA anticodon of tRNAPheis aminoacylated by phenylalanyl tRNA synthetaseof E. coli [9] This implies that thepresence of either guanosine (wild-type tRNAPhe(GAA))or cytosine (ECF suppressor tRNA (CUA)) inthe wobble position of the anticodon did not seem to influence the aminoacylation by phenylalanyl tRNA synthetase. If tRNAs with an adenosine inthe wobble position of the anticodon that can decode only the NNU codons arenot favored by E. coli during evolution, phenylalanyl tRNA synthetase may not aminoacylate tRNAPhe(AAA). We examined how a tRNAPhewith an adenosine in the wobble position of the anticodon would function in E. coli

MATERIALS ANDMETHODS

Not identified

RESULTS AND DISCUSSION

CG C C U