PEPTIDYL TRANSFERASE AND BEYOND

The peptidyl transferase center of the Escherichia coli ribosome encom passes a number of 50S-subunit proteins as well as several specific se gments of the 23S rRNA. Although our knowledge of the role that both r ibosomal proteins and 23S rRNA play in peptide bond formation has stea dily increased, the location, organization, and molecular structure of the peptidyl transferase center remain poorly defined. Over the past 10 years, we have developed a variety of photoaffinity reagents and st rategies for investigating the topography of tRNA binding sites on the ribosome. In particular, we have used the photoreactive tRNA probes t o delineate ribosomal components in proximity to the 3' end of tRNA at the A, P, and E sites. In this article, we describe recent experiment s from our laboratory which focus on the identification of segments of the 23S rRNA at or near the peptidyl transferase center and on the fu nctional role of L27, the SOS-subunit protein most frequently labeled from the acceptor end of A- and P-site tRNAs. In addition, we discuss how these results contribute to a better understanding of the structur e, organization, and function of the peptidyl transferase center.