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Category: Microbial Genetics and Molecular Biology
Translational Suppression: When Two Wrongs DO Make a Right, Page 1 of 2
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This chapter talks about the genetic translational suppression, that is, suppression caused by a mutation in the gene for one of the translational macromolecules, particularly tRNAs. Such suppressor mutants usually generate a much stronger suppression signal, more easily allowing the analysis of the alteration in translational fidelity. Furthermore, they allow the study of structural determinants of that macromolecule's functions and of its functional interactions with other translational macromolecules. Translational suppression is a most effective way to examine the structure, function, and interactions of any translational macromolecule, as long as and to the extent that that molecule is involved in the specificity or accuracy of translation. The chapter is divided into four parts: (a) review of the requirements of any system to be used for the in vivo selection and study of suppressors, (b) examples of interesting suppressors of missense, nonsense, and frameshift mutations, (c) conclusions from and ramifications of some suppressor tRNA studies, and (d) discussion of some suppressors that are not altered tRNAs but that nevertheless lead, directly or indirectly, to altered functioning of some tRNA.
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Missense suppressors derived from tRNAGly isoacceptors in E. coli
a Anticodon consists of nucleotides 34, 35, and 36. Anticodon loop is nucleotides 32 to 38. For entire wild type sequences, see reference 119 . U* is a modified form of U that is unidentified but related to S-methylaminomethyl-2-thiouridine (mam5s2U). Ux, Uy and Uz, however, are unidentified modified U's that are different from U*. t6A, N-[(9-β-D-ribofuranosylpurin-6-yl) carbamoyl] threonine. ms2i6A, 2-methylthio-N6-isopentenyladenosine.
b Each suppressor tRNA is specific for the codons listed, failing to suppress other trpA missense as well as nonsense mutations.
c Absence of references in this column is meant to indicate results obtained in my laboratory. Suppressor isolations and conversions were by F. T. Pagel, K. A. Hijazi, and myself; tRNA sequence analyses were by N. E. Prather, B. H. Mims, and myself.
d Deduced from sequencing of glyU(SuUAG) and glyU(SuUG A/G) tRNAs, each of which was obtained from it in one step (see Table 3 ).
e These tRNAs have eight nucleotides in the anticodon loop. As a result, the anticodon has been shifted from nucleotides 34, 35, and 36 to nucleotides 35, 36, and 37 ( 83 , 105 ).
Missense suppressors derived from tRNAGly isoacceptors in E. coli
a Anticodon consists of nucleotides 34, 35, and 36. Anticodon loop is nucleotides 32 to 38. For entire wild type sequences, see reference 119 . U* is a modified form of U that is unidentified but related to S-methylaminomethyl-2-thiouridine (mam5s2U). Ux, Uy and Uz, however, are unidentified modified U's that are different from U*. t6A, N-[(9-β-D-ribofuranosylpurin-6-yl) carbamoyl] threonine. ms2i6A, 2-methylthio-N6-isopentenyladenosine.
b Each suppressor tRNA is specific for the codons listed, failing to suppress other trpA missense as well as nonsense mutations.
c Absence of references in this column is meant to indicate results obtained in my laboratory. Suppressor isolations and conversions were by F. T. Pagel, K. A. Hijazi, and myself; tRNA sequence analyses were by N. E. Prather, B. H. Mims, and myself.
d Deduced from sequencing of glyU(SuUAG) and glyU(SuUG A/G) tRNAs, each of which was obtained from it in one step (see Table 3 ).
e These tRNAs have eight nucleotides in the anticodon loop. As a result, the anticodon has been shifted from nucleotides 34, 35, and 36 to nucleotides 35, 36, and 37 ( 83 , 105 ).
Missense suppressors derived from tRNAs other than tRNAGly
a The glu3 suppressor is from S. cerevisiae. All others are from E. coli.
b Anticodon consists of nucleotides 34, 35, and 36. For entire wild-type sequences, see reference 119 .
c In tRNAGlu from S. cerevisiae, U9 is mcm5s2U, i.e., 5-methoxycarbonylmethyl-2-thiouridine. In E. coli tRNALys, U8 is mam5s2U, i.e., 5-meth-ylaminomethyl-2-thiourdine. In the elongator tRNA Met of E. coli, C4 is ac4C, i.e., N4-acetylcytidine.
Missense suppressors derived from tRNAs other than tRNAGly
a The glu3 suppressor is from S. cerevisiae. All others are from E. coli.
b Anticodon consists of nucleotides 34, 35, and 36. For entire wild-type sequences, see reference 119 .
c In tRNAGlu from S. cerevisiae, U9 is mcm5s2U, i.e., 5-methoxycarbonylmethyl-2-thiouridine. In E. coli tRNALys, U8 is mam5s2U, i.e., 5-meth-ylaminomethyl-2-thiourdine. In the elongator tRNA Met of E. coli, C4 is ac4C, i.e., N4-acetylcytidine.
Termination codon suppressors of trpA amber (UAG), ochre (UAA), and opal (UGA) mutations in E. coli
a Anticodon consists of nucleotides 34, 35, and 36. Anticodon loop is nucleotides 32 to 38. For entire wild-type sequences, see reference 119 . In tRNAGly2, U* represents an unidentified modified U; in tRNAGly 1, Ux and U y are unidentified modifications of U that differ from U*; in tRNALys and tRNAGly 2, U8 is 5-methylaminomethyl-2-thiouridine (mam5s2U). ms2i6A, 2-methylthio-N6-isopentenyladenosine.
b Each suppressor is specific for the codons listed, failing to suppress other trpA nonsense as well as missense mutations.
c Absence of references in this column is meant to indicate results obtained in my laboratory. Suppressor isolations and conversions were by F. T. Pagel, K. A. Hijazi, and myself; tRNA sequence analyses were by N. E. Prather, B. H. Mims, and myself.
d This tRNA has eight nucleotides in the anticodon loop. See Table 1 , footnote e.
Termination codon suppressors of trpA amber (UAG), ochre (UAA), and opal (UGA) mutations in E. coli
a Anticodon consists of nucleotides 34, 35, and 36. Anticodon loop is nucleotides 32 to 38. For entire wild-type sequences, see reference 119 . In tRNAGly2, U* represents an unidentified modified U; in tRNAGly 1, Ux and U y are unidentified modifications of U that differ from U*; in tRNALys and tRNAGly 2, U8 is 5-methylaminomethyl-2-thiouridine (mam5s2U). ms2i6A, 2-methylthio-N6-isopentenyladenosine.
b Each suppressor is specific for the codons listed, failing to suppress other trpA nonsense as well as missense mutations.
c Absence of references in this column is meant to indicate results obtained in my laboratory. Suppressor isolations and conversions were by F. T. Pagel, K. A. Hijazi, and myself; tRNA sequence analyses were by N. E. Prather, B. H. Mims, and myself.
d This tRNA has eight nucleotides in the anticodon loop. See Table 1 , footnote e.
Modified nucleosides” in the anticodon loops of related E. coli suppressor tRNAs
a A2,2-methyladenosine, m2A; A5, 2-methylthio-N6-isopentenyladenosine, ms2i6A; A7, N-[(9-beta-D-ribofuranosylpurin-6-yl) carbamoyl] threonine, t6A; U8, 5-methylaminomethyl-2-thiouridine, mam5s2U; U*,Ux,Uy,Uz, unidentified modifications of U that are different from U8.
b See Tables 1 and 3 for further information.
c Brackets indicate that the modification is partial. Parentheses indicate that the nucleoside is predominantly unmodified. Absence of brackets or parentheses indicates that the nucleoside is either completely unmodified at the position in question (no modified nucleoside detected) or completely modified (no unmodified nucleoside detected at precisely that position). These observations are reported by comparison with or in contrast to modified or unmodified nucleosides present in relevant wild type tRNAs (for example, gly2, trp, cys, or glu) from cells grown under the same conditions.
d These suppressor tRNAs have an extra A (A38:A) (not shown) in the anticodon loop, resulting in an anticodon shift to nucleotides 35, 36, and 37 ( 83 , 105 ).
e All of these lysT derivatives retain the C70 to U change in the amino acid acceptor stem.
f Corresponds to both supG and supL (see Table 3 ).
Modified nucleosides” in the anticodon loops of related E. coli suppressor tRNAs
a A2,2-methyladenosine, m2A; A5, 2-methylthio-N6-isopentenyladenosine, ms2i6A; A7, N-[(9-beta-D-ribofuranosylpurin-6-yl) carbamoyl] threonine, t6A; U8, 5-methylaminomethyl-2-thiouridine, mam5s2U; U*,Ux,Uy,Uz, unidentified modifications of U that are different from U8.
b See Tables 1 and 3 for further information.
c Brackets indicate that the modification is partial. Parentheses indicate that the nucleoside is predominantly unmodified. Absence of brackets or parentheses indicates that the nucleoside is either completely unmodified at the position in question (no modified nucleoside detected) or completely modified (no unmodified nucleoside detected at precisely that position). These observations are reported by comparison with or in contrast to modified or unmodified nucleosides present in relevant wild type tRNAs (for example, gly2, trp, cys, or glu) from cells grown under the same conditions.
d These suppressor tRNAs have an extra A (A38:A) (not shown) in the anticodon loop, resulting in an anticodon shift to nucleotides 35, 36, and 37 ( 83 , 105 ).
e All of these lysT derivatives retain the C70 to U change in the amino acid acceptor stem.
f Corresponds to both supG and supL (see Table 3 ).