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Inhibition of lacZ Gene Translation Initiation in trp-lac Fusion Strains

¹ Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-Madison, Madison, Wisconsin 53706; Department of Bacteriology and Immunology, Harvard Medical School, Boston, Massachusetts 02115; Department of Biological Sciences, Columbia University, New York, New York 10027; and Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

ABSTRACT

Different levels of ß-galactosidase are found in various trp-lac fusion strains. These levels of ß-galactosidase fall within a 60-fold range. The amount of thiogalactoside transacetylase activity detected in these same strains only varies 10-fold and is found in amounts greater than those predicted from the ß-galactosidase levels. The observation that the ß-galactosidase and thiogalactoside transacetylase levels are not directly proportional, that the lacZ messenger ribonucleic acid (mRNA) levels are not proportional to the ß-galactosidase activity, that, at least for the one fusion strain tested, the SuA polarity suppressor does not affect the ß-galactosidase level, and that, in all but one strain, the ß-galactosidase activity appears to reside in normal ß-galactosidase molecules suggests that the disproportionately low production of ß-galactosidase is due to a decrease in the frequency of translation initiation of lacZ mRNA in these strains. Several mechanisms are proposed to explain this decrease. Some possible bases for the disproportional production of ß-galactosidase and thiogalactoside transacetylase are also described. The preferred explanation for these disproportional enzyme levels is that only a fraction of the full complement of ribosomes need initiate translation at lacZ for the functional synthesis of lac mRNA to occur and that once the lac ribonucleic acid is made a full complement of ribosomes can bind at internal translation initiation sites at Y and A.

¹ Present address: Department of Biology, Queens College of the City University of New York, Flushing, N.Y. 11367.

² Present address: Department of Biochemistry, University of Pittsburgh, Pittsburgh, Pa. 15261.

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