The First RNA Polymerase to Be Discovered Did Not Require a DNA Template

The first enzyme discovered that could catalyze polynucleotide synthesis was a bacterial enzyme called polynucleotide phosphorylase. This enzyme, isolated by Severo Ochoa and Marianne Grunberg-Manago in 1955, could make long chains of 5 -3 -linked polyribonucleotides starting from nucleoside diphosphates. However, there was no template requirement for this synthesis, and the sequence was uncontrollable except in a crude way by adjusting the relative concentrations of different nucleotides in the starting materials. 

Shortly after Ochoa s studies, Sam Weiss began a search for a DNA-directed RNA polymerase. His experimental design was influenced by the theory that RNA must be made on a DNA template if it is to carry the genetic message. He was also influenced by Komberg s discovery that DNA synthesis required nucleoside triphosphates for substrates rather than nucleoside diphosphates. With crude liver extracts, Weiss was able to demonstrate a capacity for RNA synthesis that was severely inhibited by DNase. Weiss s results touched off systematic investigations of RNA metabolism in many laboratories. A continuous expansion of research effort from that time on has yielded a wealth of understanding about the transcription process and related aspects of RNA metabolism. 

In eukaryotic cells the nuclear membrane separates the processes of RNA and protein synthesis. This can be demonstrated with radioactive substrates that are precursors of RNA and protein. Immediately after exposure of cells to labeled precursors, the RNA label becomes fixed in the nucleus, and the protein label becomes fixed in the cytoplasm. Eventually most of the labeled RNA becomes transferred to the cytoplasm, and a fraction of the labeled protein becomes transferred to the nucleus. 

Exposure of cells to radioactively labeled RNA and protein precursors 

RNA label first becomes fixed in nucleus while protein label first becomes fixed in cytoplasm 

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