Studying gene expression for a long time

Abstract

In the 1960s and 70s before the recombinant DNA era we studied the structure and expression of the repetitive Xenopus laevis ribosomal RNA genes and 5S ribosomal RNA genes. They were isolated by physical and chemical means from genomic DNA. These genes revealed transcribed and nontranscribed spacers, pseudogenes, horizontal evolution of tandem repeating genes including the conservation of functional sequences and the divergence of spacer regions. The Xenopus oocyte accumulates enormous numbers of ribosomes for a single cell. The solution for this demand for quantity differs for each component. The genes encoding the large 18S and 28S RNAs are amplified by about 1000 times. This was the first instance of specific gene amplification. The 5S RNA genes are not amplified but encoded in a huge family of about 20,000 members expressed only in the oocyte. The purified 5S RNA genes were convenient for studying gene expression in vitro revealing the DNA signals for initiation and termination of transcription. In 1990 I changed my research to a more complex problem of gene expression, amphibian metamorphosis. Essentially every cell type in each tadpole organ is targeted for change by thyroid hormone (TH). Some organs (for example the limb) complete differentiation by multiple cell autonomous programs. Others (intestine and pancreas) have TH-induced cellcell interaction. This biological problem has not yet revealed its most exciting secrets. Many scientists have contributed to my research. It was my greatest good fortune to have discovered the Carnegie Institution. I am extremely grateful to the NIGMS and the Mathers Foundation for long term funding.