Expression of Nuclear Genes -

The synthesis of a functional product within a cell, using the information contained in the coding sequence of a gene, relies on the coordination of a whole series of events. If the gene codes for a polypeptide (rather than a ribosomal or transfer RNA molecule), it must be transcribed into messenger RNA, which is then translated to give that polypeptide processing of both the mRNA and polypeptide molecules may occur.

There is potential for regulation of gene expression at all these steps and elucidation of the mechanisms and regulatory steps involved has been the subject of intensive investigation. The study of animal systems in this respect has always been more advanced than that of plants and much of the information learned from animals can be applied to and assist in the investigation of plant gene expression.

In contrast to the major part of the chromosomes, these puffs are transcriptionally active. They have an appearance and staining behaviour that suggests a different physical structure from the inactive regions. More evidence for activation comes from studies of nuclease sensitivity of chro­matin.

Small regions preferentially digested by DNase , an enzyme that hydrolyzes deoxyribosephosphate bonds in double stranded DNA. It seems likely that these regions are more open and thus more exposed to the enzyme. Some of these regions have been associated with active gene expression. In chicken red blood cells, globin genes (actively expressed) are six times more sensitive to DNase I than are ovalbumin genes (which are not expressed).

Activation of nuclear genes is influenced by enhancers. While it is not clear what they do, it is possible that they help put the adjacent genes into a physical state appropriate for transcription. Enhancers have been associated with two other physical characteristics of DNA: the Z-helical form and methylation. There are other changes in the genes that may precede transcription in differentiating systems. One involves rearrangements.

These are seen most clearly in cells that produce immunoglobulins (antibodies). Another process related to activation is amplification.
In this process the genes are replicated many times and thus multiple templates are available fbr transcription. As is found in gene rearrangement, this is not a common process but several examples are known. One example involves the genes, for ribosomal RNA in the macronucleus of Tetrahymena, which are multiple copies of the genes in the micronucleus. Another example is the adapta­tion of animal cells to the antibiotic methotrexate: Amplification of the gene for dihydrofolate reductase (the enzyme inhibited by methotrexate) provides more template and thus more mRNA and enzyme. With additional enzyme the cells develop some resistance to a given amount of the antibiotic. In yet another example, amplification of an esterase gene, which leads to high levels of a detoxifying enzyme, makes mosquitos resistant to organophosphorus insecticides.