Ptashne, Mark (1940– )

US molecular biologist who has contributed much to our understanding of the mechanisms of transcription (formation of messenger RNA from DNA) in prokaryotic cells, such as bacteria or cyanobacteria. He has worked for many years on the mechanisms that switch genes on and off.

In genetic studies using the bacterium Escherichia coli, he postulated repressor proteins might be produced within a cell to bind to specific sites on or close to the gene to inactivate it. In one set of elegant studies, he showed that the gene for the enzyme b-galactosidase is turned off by a repressor molecule when lactose is not present. Ptashne showed that the repressor prevents transcription by binding to a specific sequence of the DNA called the operator, thus preventing the activity of the enzymes necessary for the transcription of neighbouring DNA sequences. This process can be reversed by the presence of lactose in the cell.

Ptashne was born in Chicago and obtained his BA and PhD at Reed College and Harvard University respectively. He joined the staff of the biochemistry department at Harvard 1968 and was elected professor of biochemistry 1971.

Ptashne's work acted as the first model for the reversible activity of repressor proteins in the control of transcription and was the forerunner for subsequent studies showing the involvement of many other such molecules in gene expression.