Switching-on genes in development.

Switching-on genes in development

Studies of the simplest gene system in plants and animals are drastically changing scientists' ideas of how genes work in complex organisms, Donald D. Brown of the Carnegie Institution in Baltimore reported last week at the National Institutes of Health in Bethesda, Md. Whether these genes are active or silent, he has found, depends both on the folding of DNA with proteins into its characteristic "chromatin' structure and on the stable binding of particular proteins to a site in the center of the gene. This mechanism of gene control is quite different from that of bacteria, which previously was the only such mechanism described at this level of detail. Gene regulation is a basic puzzle of modern biology, with implications for all aspects of how organisms function.

Brown and his colleagues studied two families of genes found in the African clawed toad, Xenopus laevis. Each gene encodes a small RNA molecule, called 5S ribosomal RNA, which is part of the cellular organelle that makes protein. The two families of genes are called the oocyte (egg cell) genes and the somatic (body) genes. The families differ in about six positions among the 120 nucleotides that make up each gene.

In the toad egg cell, or oocyte, all of the 5S ribosomal RNA genes are active. But because there are 20,000 oocyte genes and only 400 somatic genes, the oocyte form of 5S ribosomal RNA predominates. In contrast, in somatic cells of the toad, the somatic genes are 1,000 times as active as the oocyte genes.

A two-tiered system governs the activity of the oocyte gene, Brown reports. The top tier involves chromatin, the natural chromosomal structure in which the DNA is condensed with proteins called histones. Brown's team has developed a new test that measures the activity of chromatin, rather than just naked DNA. When the chromatin from somatic cells is dipped into a solution containing all the required components, the somatic genes are expressed and the oocyte genes remain repressed, as in the intact cell.

The scientists next disrupted the chromatin structure, dissociating the DNA from the histone H1. The result was a massive synthesis of the oocyte form of 5S ribosomal RNA. Brown concludes that the repressed state of this gene and others is maintained by the interaction between DNA and histone H1.

The second tier of gene control relies on three proteins that Brown calls transcription factors A, B and C. These proteins must bind to the center of the gene, forming a "transcription complex,' before the enzyme called polymerase III begins making new RNA.

The surprising finding about this transcription complex is its stability. It remains in place for many rounds of RNA synthesis. Somehow the complex avoids being knocked off the DNA as the polymerase works its way along the gene. "The polymerase goes through the transcription complex as if it were butter,' Brown says.

In recent experiments, Brown and his colleagues demonstrated that the presence of a transcription complex underlies the specific activity of the oocyte gene. In the region where the factors bind, the oocyte and somatic genes differ by three nucleotides out of 50. The A factor, they find, binds more strongly to the somatic than to the oocyte gene. This discrimination is most evident in situations where there is limited factor. In the oocyte there are 10,000,000 factor A molecules per 5S ribosomal RNA gene, but in the somatic cell there is only one factor A molecule for every five of these genes.

The intriguing question now is whether the transcription complex is the "memory' that maintains the activity state of the gene from one cell generation to the next. If so, it might be the basis by which-- as an organism differentiates--various cell lines become committed to expressing different patterns of gene activity.