Structure and synthesis of the ribosomal ribonucleic acid of prokaryotes

Abstract

The ribosomes of phylogenetically diverse prokaryotes are superficially similar in their properties. The collections of ribosomal proteins from diverse prokaryotes are similar in the numbers and sizes of their components, but the corresponding proteins from different organisms do not necessarily display close antigenic relationships. Therefore, the prokaryotes have diverged considerably in the detailed structures of the components of their ribosomes. Nevertheless, functional homology has been retained: hybrid ribosomal subunits which are active in protein synthesis may be reconstituted from the component proteins and rRNA from little related organisms. The nucleotide compositions of the total rRNA from diverse prokaryotes are remarkably constant, even though the base compositions of their genomes vary widely. The individual rRNA classes of a given strain of bacteria are largely homogeneous in their structures, but minor differences, in only a few nucleotides, are evident. These presumably occur because each organism possesses multiple copies of the rRNA genes, and their individual structures are evolving more or less independently. There is no evidence for functional heterogeneity within the rRNA population of individual prokaryotic organisms. Most prokaryotes contain multiple genes specifying the rRNA components. Generally, about 0.3 to 0.4% of the total genome DNA is complementary to rRNA. The lack of appropriate mutants in the rRNA genes has retarded locating them within the linkage map of E. coli, but at least most of the rDNA loci now were mapped by DNA rRNA hybridization experiments using episomes which span defined regions of the chromosome.