Polarity and enzyme functions in mutants of the first three genes of the tryptophan operon of Escherichia coli.

N operons which are believed to be transcribed into polycistronic messenger I RNA molecules, some revertible mutations result in a lowered rate of synthesis of enzymes specified by genes operator-distal to the mutated gene. This phenomenon, termed polarity (FRANKLIN and LURIA 1961 ; JACOB and MONOD 1961 ) , has been studied in several organisms and a variety of explanations have been proposed for the observed effect on enzyme levels. Mutational changes which cause polarity are now known to have in common the introduction of a polypeptide chain-termination codon within a structural gene of an operon (NEWTON et al. 1965; WHITFIELD, MARTIN and AMES 1966; YANOFSKY and ITO 1966). Mutations of the missense type, resulting in amino acid substitutions, do not cause polarity. In the present study mutants of the first three genes of the tryptophan operon of Escherichia coli were characterized. The properties of the mutants and their respective altered proteins or protein fragments revealed interesting features of the corresponding wild-type proteins. Many of the mutants exhibited polarity and were employed in determinations of the shape of the polarity gradient for their respective genes. Additionally, antipolarity ( ITO and CRAWFORD 1965; YANOFSKY and ITO 1967), an effect on expression of the gene immediately preceding a gene with a chain termination mutation, was examined in strong polar mutants of several genes of the operon.