Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the n

ID: 150050 • Letter: E

Question

Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the nature of tryptophan synthesis in E coli. In one of Yanofsky's experiments, he identified glycine (Gly) as the wild-type amino acid in position 211 of tryptophan synthetase, the product of the trpA gene. He identified two independent missense mutants with defec- tive tryptophan synthetase at these positions that resulted from base-pair substitutions. One mutant encoded arginine (Arg) and another encoded glutamic acid (Glu). At position 235, wild-type tryptophan synthetase contains serine (Ser), but a base-pair substitution mutant encodes leucine (Leu). At position 243, the wild-type polypeptide contains gluta- mine, and a base-pair substitution mutant encodes a stop codon. Identify the most likely wild-type codons for posi- tions 211, 235, and 243. Justify your answer in each case.

Explanation / Answer

Gly is encoded by GGU, GGC, GGA, and GGG.

Arg is encoded by CGU, CGC, CGA, CGG, AGA and AGG. From these it is clear one of the following could cause a Gly to Arg mutation.

1) Gly is coded by either GGA or GGG, and the first letter G changes to A.

2) Gly is coded any of the four codons and the first letter G changes to C.

In another mutant, Gly --> Glu was observed at position 211.

Since Glu is coded by GAA and GAG, only if Gly is coded by either GGA or GGG, can a single base mutation at the second letter (G--> A) could lead to the observed mutation.

So, at position 211, Gly must be coded by GGA or GGG.

Ser is coded by UCU, UCC, UCA, UCG, AGU and AGC.

Leu is coded by UUA, UUG, CUU, CUC, CUA, and CUG. Just as we did with Gly211, compating the codes, we can realize that two codons of Serine UCA and UCG differ from Leucine's UUA and UUG by only one base. A mutation in the second letter (C--> U) can produce a Ser--> Leu mutation.

So, Ser235 must be coded by either UCA or UCG.

Glutamine is coded by CAA and CAG. A mutation at the first letter could lead to stop codons UAA and UAG. So, CAA and CAG are both likely to be the codon for Gln at 243.

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Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the n

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Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the n

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