DNA sequencing and gene structure.

When we work out the structure of DNA molecules, we examine the fundamental level that underlies all process in living cells. DNA is the information store that ultimately dictates the structure of every gene product, delineates every part of the organism. The order of the bases along DNA contains the complete set of instructions that make up the genetic inheritance. We do not know how to interprete those instructions; like a child, we can spell out the alphabet without understanding more than a few words on a page. I came to the chemical DNA sequencing by accident. Since the middle sixties my work had focussed on the control of genes in bacteria, studying a specific gene product, a protein repressor made by the control gene for the lac operon (the cluster of genes that metabolize the sugar lactose. Benno Müller-Hill and I had isolated and characterized this molecule during the late sixties and demonstrated that this protein bound to bacterial DNA immediately at the beginning of the first gene of the three-gene cluster that this repressor controlled (1, 2). In the years since then, my laboratory had shown that this protein acted by preventing the RNA polymerase from copying the lac operon genes into RNA. I had used the fact that the lac repressor bound to DNA at a specific region, the operator, to isolate the DNA of this region by digesting all of the rest of the DNA with DNase to leave only a small fragment bound to the repressor, protected from the action of the enzyme. This isolated a twenty-five base-pair fragment of DNA out of the 3 million base pairs in the bacterial chromosome. In the early seventies, Allan Maxam and I worked out the sequence of this small fragment (3) by copying this DNA into short fragments of RNA and using on these RNA copies the sequencing methods that had been developed by Sanger and his colleagues in the late sixties. This was a laborious process that took several years. When a student, Nancy Maizels, then determined the sequence of the first 63 bases of the messenger RNA for the lac operon genes, we discovered that the lac repressor bound to DNA immediately after the start of the messenger RNA (4), in a region that lies under the RNA polymerase when it binds to DNA to initiate RNA synthesis. We continued to characterize the lac operator …