Engineering of bacterial ribosomes: replacement of all seven Escherichia coli rRNA operons by a single plasmid-encoded operon.

Abstract

Since the discovery of the ribosome as the machinery essential for protein synthesis in the mid-1950s, extensive studies have been carried out with respect to ribosomal structure, function, biosynthesis, regulation. For obvious historical reasons, these studies, especially those on the structure–function relationship, have been done mostly by using Escherichi coli, leading to enormous amounts of information on E. coli ribosomes (1). One of the most impressive recent developments is the demonstration in vitro of the peptidyl transferase activity of rRNA with few or no proteins attached, confirming the suspected essential roles of rRNAs in ribosomal functions (2–4). In parallel to the in vitro studies of E. coli ribosomes, extensive genetic and physiological studies have also been carried out, identifying all of the genes for ribosomal components and giving insight into mechanisms by which E. coli regulates the synthesis of ribosomes and their molecular components (5, 6). Genetic approaches have always been essential to test the validity of conclusions derived from in vitro experiments regarding ribosome functions or regulation of ribosome synthesis. In the article by Asai et al. published in this issue of the Proceedings (7), Squires and coworkers describe their success in constructing an E. coli strain (“Δ7 prrn”) in which each of the seven chromosomal rRNA operons is inactivated by a deletion spanning the 16S and 23S RNA-coding regions, and rRNA is transcribed from a single rRNA operon carried by a multicopy plasmid. Although this success does not provide major surprises, the newly constructed rrn deletion strains provide a powerful system for mutational analysis of the structure and function of rRNAs as well as a system for studying the significance of the presence of multiple rRNA operons in bacteria.