The recent development of new genetic systems for the construction of rRNA and ribosomal protein mutants in Escherichia coli has facilitated both structural and functional analyses of these molecules. Mutations in ribosomal proteins resulting in antibiotic resistance or alterations in the accuracy of translation can now be interpreted in structural terms while the consequences for ribosome structure of alterations in rRNA have been visualized directly for the first time by cryo-electron-microscopic techniques. This chapter reviews the recent developments that the author's have made in this area. They describe some of the recent applications of this strain and its potential for further analysis of rRNA function. The acceptance of the base-pairing model of enhancer action has led to the conclusion that all downstream box (DB) elements enhance translation by the same mechanism and has prompted the identification of DB elements by 16S rRNA sequence complementarity alone. Dihydrouridine (D) is one of the most common posttranscriptional modifications found in bacterial and eukaryal tRNAs, where it is believed to allow conformational flexibility in the loop regions of tRNAs that are involved in tertiary interactions. The development of an E. coli strain that permits the expression of pure populations of mutant rRNAs has greatly facilitated genetic, functional, and structural analyses of rRNA.