Ribonucleic acid-permeable mutant of Escherichia coli

Abstract

An RNA-permeable mutant was isolated from a tryptophan amber auxotrophic strain of Escherichia coli after mutagenesis with N-methyl- N'-nitro- N-nitrosoguanidine. The rationale of the isolation was based on the suppression of an amber mutation. A strain was selected, which could grow in minimal medium supplemented with transfer RNA prepared from an suI-carrying strain but not from an su − strain. This mutant incorporated 3H-labeled bulk RNA into the cells at a rate 40 times higher than did the parent strain. The level of tryptophan requirement, susceptibility to the lytic action of lysozyme and RNase activity in the culture medium of the mutant strain did not differ from those of the parent strain. The mutant strain incorporated 3H-labeled ribosomal RNA equally as well as it incorporated 3H-labeled transfer RNA and the incorporation was competitively inhibited by any species of cold RNA. However, the fate of 3H-labeled rRNA after incorporation resulted in degradation to yield acid-soluble fragments whereas tRNA after incorporation remained intact in the cell. From these results, it was concluded that the mutant selected on the basis of amber suppression could actually incorporate tRNA molecules without loss of their functions.