Yanofsky, Charles

The elegant genetic studies by the group of Charles Yanofsky at Stanford University, conducted before the crystal structure was known, confirm this mechanism. The side chain of Ala 77, which is in the loop region of the helix-turn-helix motif, faces the cavity where tryptophan binds. When this side chain is replaced by the bulkier side chain of Val, the mutant repressor does not require tryptophan to be able to bind specifically to the operator DNA. The presence of a bulkier valine side chain at position 77 maintains the heads in an active conformation even in the absence of bound tryptophan. The crystal structure of this mutant repressor, in the absence of tryptophan, is basically the same as that of the wild-type repressor with tryptophan. This is an excellent example of how ligand-induced conformational changes can be mimicked by amino acid substitutions in the protein. 

Among the genes concerned with biosynthetic processes, those uniquely involved in synthesis of the amino acid tryptophan are possibly the best understood. This is mostly a result of the efforts of Charles Yanofsky and his colleagues, who have used a wide variety of genetic and biochemical techniques to probe the complexities of this system. 

A novel mechanism for regulating transcription in bacteria was discovered by Charles Yanofsky and his colleagues as a result of their studies of the tryptophan operon. The 7-kb mRNA transcript from this operon encodes five enzymes that convert chorismate into tryptophan (Section 24.2.10). The mode of regulation of this operon is called attenuation, and it... 

A second level of control of the tryptophan biosynthetic pathway was discovered by Charles Yanofsky when he characterized mutants in the trp operon that did not affect Trp repressor binding. Yanofsky and his colleagues characterized a novel form of transcriptional control they called attenuation, which depends on the unique linkage between transcription and translation in prokaryotes. As shown in Figure 28.11, the intracellular concentration of TRP-tRNATrp determines if the ribosome will pause at a set of codons in the trp mRNA that specify consecutive Trp residues. When tryptophan levels are high, and TRP-tRNATrp is available, then the transcriptional termination hairpin loop forms and RNA polymerase disengages from the DNA template just downstream of a polyuridine... 

In order to prove that regulation by attenuation occurs in vivo, Charles Yanofsky and others studied tryptophan synthesis regulation in a series of E. colt mutants. For each mutant described below predict the expression of tryptophan synthesis genes in the presence or absence of tryptophan. 

In the course of these studies, the important discovery of the degeneracy of the code emerged. This pioneer work by Ochoa and Nirenberg was extended and refined by H. Gobind Khorana, Charles Yanofsky and others. By 1966 the composition of the genetic code was almost completely elucidated, a major conquest in the history of science. 

My close friendship with Charles Yanofsky and the times we spent together revealed to me the power of genetics for probing structure-function relationships of enzymes. Consequently, Bill Folk, Maurizio laccarino and I examined mutational alterations affecting amino acyl tRNA synthetases and John Carbon, Charles Hill, Larry Soli, Folk and Moshe Yaniv studied genetically altered tRNA " " the latter investigations proved to be the most productive because they showed that changes in nucleotide sequences of tRNA could affect the specificity of amino acylation and the translation of codons in mRNA. ... 

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