The trpB and trpA coding regions of the polycistronic trp mRNA of Escherichia coli are separated by overlapping translation stop and start codons. Efficient translation of the trpA coding region is subject to translational coupling, i.e., maximal trpA expression is dependent on prior translation of the trpB coding region (1,2). Previous studies demonstrated that the trpA Shine-Dalgarno sequence (within trpB) and/or the location of the trpB stop codon influenced trpA expression (2). To examine the effect of stop codon location specifically, we constructed plasmids in which different nucleotide sequences preceding the trpA start codon were retained, and only the reading frame was changed. When trpB translation proceeded in the wild type reading frame and terminated at the normal trpB stop codon, trpA polypeptide levels were elevated over the levels observed when translation stopped before or after the natural trpB stop codon. The proximity of the trpB stop codon to the trpA start codon therefore markedly influences trpA expression.
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ACKNOWLEDGEMENTS The authors are indebted to Virginia Horn, Mark Mersereau, Janet Paluh and Gregory Pazour for their assistance with this investigation. They are also indebted to Dr. Deborah Steege of Duke University for helpful suggestions and for permission to cite her paper prior to publication. They also thank Paul Gollnick and James Roesser for their helpful comments on the manuscript. Larry Gold's special insight into translation initiation events is greatly acknowledged. These studies were supported by an American Cancer Society Junior Faculty Research Award (JFRA-171) to A.D., and by grants from the National Science Foundation (DMB 8703685) and the American Heart Association (69-015) to C. Y. A.D. is a McKnight Land-Grant Professor at the University of Minnesota and C. Y. is a Career Investigator of the American Heart Association.