Chapter X: Polarity and Translational Punctuation

Abstract

INTRODUCTION The three structural proteins of the lac operon are produced in different molar ratios. While the mechanism which gives rise to this natural polarity is still unknown, the complex phenomena of mutational polarity, with its relationship to polypeptide initiation and termination, seems to be the most likely direction from which an understanding of natural polarity will come. In this chapter the current status of work on the mechanism of polypeptide initiation and termination will be reviewed briefly. Then I will discuss in detail the phenomena of mutational polarity in the lac operon and its relationship to polypeptide initiation and termination. INITIATION Each separate polypeptide chain synthesized by an operon has an initiation code sequence. A complex mechanism is required to recognize this sequence and initiate polypeptide synthesis at the correct site and rate and in the right reading frame. The nature of the code sequence and the mechanism of initiation is under very active study using a variety of techniques. The current status of this work is summarized in the scheme shown in Fig. 1 (see Kolakofsky, Dewey, and Thach, 1969 for more detailed review and bibliography). A 30 S ribosomal subunit, an F-met charged tRNA f , and a messenger RNA react in the presence of GTP and 3 protein factors f1, f2 and f3. The GTP is split in the reaction and the product is a multi-molecular complex containing 30 S ribosome-F-met-tRNA f -mRNA-f1. In the second step of the initiation reaction a 50 S ribosomal subunit is added to this...