The Organization of tRNA Genes

Abstract

Discussion of the organization of tRNA genes may be divided into two general topics. First, it is of interest to determine the individual primary transcription units for a number of tRNA genes in a variety of organisms. tRNA is an ideal transcription product to assay because it is stable in crude extracts. In addition, a good deal of information has already been acquired from studying tRNA transcription. As the following ten chapters show, much more information becomes available with continued investigation of tRNA transcription.

As discussed by Mazzara and McClain (this volume), all tRNA molecules are matured from the de novo transcript in a series of RNA processing reactions. The primary transcription unit for a particular tRNA must be known before it is possible to define all of the processing reactions that are involved in producing the mature tRNA. This information is available only for several tRNAs in Escherichia coli. In particular, the organization of the tRNA₁^Tyr transcription unit has been studied in detail. Rossi et al. (this volume) review that work. Not only is the transcription unit known for this gene, but through the work of Khorana and his collaborators (Ryan et al., this volume) the gene has been chemically synthesized and shown to function in vivo. This work has raised a puzzling and very interesting question. Cells harboring a plasmid carrying the synthetic tRNA₁^Tyr gene produce enhanced levels of tRNA^Tyr (as expected, since there are a larger number of gene copies). In contrast, cells harboring a plasmid containing...