From comparative analyses of the nucleotide sequences of genes encoding ribosomal RNAs and several proteins, molecular phylogeneticists have constructed a "universal tree of life," taking it as the basis for a "natural" hierarchical classification of all living things. Although confidence in some of the tree's early branches has recently been shaken, new approaches could still resolve many methodological uncertainties. More challenging is evidence that most archaeal and bacterial genomes (and the inferred ancestral eukaryotic nuclear genome) contain genes from multiple sources. If "chimerism" or "lateral gene transfer" cannot be dismissed as trivial in extent or limited to special categories of genes, then no hierarchical universal classification can be taken as natural. Molecular phylogeneticists will have failed to find the "true tree," not because their methods are inadequate or because they have chosen the wrong genes, but because the history of life cannot properly be represented as a tree. However, taxonomies based on molecular sequences will remain indispensable, and understanding of the evolutionary process will ultimately be enriched, not impoverished.

https://science.sciencemag.org/content/sci/284/5423/2124.full.pdf

Phylogenetic Classification and the Universal Tree

https://mmbr.asm.org/content/mmbr/47/3/410.full.pdf

Linkage map of Salmonella typhimurium, edition VII.

Resistance to a drug acting on the 30S ribosomal subunit is attributed to non-methylation of 16S RNA.

Change in Methylation of 16S Ribosomal RNA Associated with Mutation to Kasugamycin Resistance in Escherichia coli

I have not mentioned the remarkable progress made mainly by Fellner and his co-workers (86) in the elucidation of the primary structure of rRNA9s and by Wittmann and his co-workers (87) in determining the structure of several ribosomal proteins. Such knowledge of primary structures is certainly the basis of complete understanding of the structure of the ribosome. With the current progress in technology, complete elucidation of the primary structure of all the ribosomal components is probably a matter of time. As indicated in this article, a rough approximation of the three-dimensional structure of ribosomes is likely to emerge soon. Although not mentioned in this article, studies of ribosomes from higher organisms are also progressing. We must, therefore, consider what further studies should be conducted and what kinds of questions we would like to solve. Some groups of investigators aim to elucidate the complete three-dimensional structure of ribosomes and to find out how these complex cell organelles function; they hope to determine the conformational changes of many of the component molecules within the ribosome structure in response to external macromolecules and cofactors engaged in protein synthesis. Such knowledge will also be important in enabling us to understand the regulation of translation of genetic messages. Other groups of investigators aim to elucidate the complex series of events which originate in the transcription of the more than 60 genes and culminate in the formation of the specific structure of the organelle. Complete reproduction in vitro of all the assembly events that occur in vivo should not be difficult to achieve in principle. It should then become possible to study in vitro any factor regulating the biogenesis of the organelle. Although we do not know whether such studies would reveal any new fundamental principle that governs the complex circuits of interconnected macromolecular interactions, the achievement of such a complete in vitro system would represent a necessary step in the comprehensive understanding of biogenesis of organelles, and eventually, of the more complex behavior and genesis of cells (89).

https://science.sciencemag.org/content/sci/179/4076/864.full.pdf

Assembly of bacterial ribosomes.

Current linkage map of Salmonella typhimurium.

Intergeneric mating between Escherichia coli and Salmonella typhosa was used to locate at least three 30S ribosomal proteins near the streptomycin locus in the region of 54 to 66 min of the E. coli map. This procedure utilizes differences in the electrophoretic patterns of 30S ribosomal protein of the parents. The results show that cistrons for 30S proteins of E. coli can replace those of S. typhosa in the Salmonella genome. Moreover, in a diploid hybrid with a Salmonella endogenote and an E. coli exogenote, both sets of cistrons are expressed.

Chromosomal Location of Ribosomal Protein Cistrons Determined by Intergeneric Bacterial Mating

The chemical and genetic structure of bacterial ribosomes.

Genetic mapping of ribosomal protein cistrons of Salmonella typhimurium and Escherichia coli was performed by phage P1 mediated, generalized transduction. From an E. coli hybrid strain which carried a S. typhimuirum F′ factor, an E. coli strain was constructed which had integrated S. typhimurium genetic material including the region of the strA locus. Salmonella genetic material from this hybrid was transduced into E. coli recipients. The ribosomal protein electrophoretic patterns of these hybrid transductants were correlated with the presence of markers contributed by each parent. The results of these studies indicate that cistrons for at least three characteristic S. typhimurium and two E. coli 30 S ribosomal proteins are closely linked to the strA locus on the genetic maps of both organisms. At least one cistron coding for a 50 S ribosomal protein is also closely linked to this locus on both maps. These findings support the concept that cistrons coding for the ribosomal proteins are clustered in one area of the genome. Mutations to spectinomycin and streptomycin resistance are closely linked in S. typhimurium and are located at strA . Images

Dennis M. O'neil

Papers

Chromosomal Location of Ribosomal Protein Cistrons Determined by Intergeneric Bacterial Mating

The chemical and genetic structure of bacterial ribosomes.

Cotransduction of strA and Ribosomal Protein Cistrons in Escherichia coli-Salmonella typhimurium Hybrids