University of Stavanger - Norway - The biological clock - PhD thesis by Melinda K. Christensen

Melinda K. Christensen, formerly Moe, defends her PhD thesis on April 13, 2007 at the University of Stavanger, in Stavanger, Norway which in entitled “A frequency independent nitrate reductase rhythm in Neurospora crassa”. The topic of her obligatory lecture was "Protein degradation as an important element of cellular regulation".

Melinda K. Christensen’s work is concerned with circadian rhythms (popularly called “The biological clock”) which play an important role in humans and many other organisms. They regulate daily and seasonal processes, among others; sleep, flowering in plants, bird migration, activity rhythms in insects, hibernation etc. As an object of study, Melinda has used the filamentous fungi, Neurospora crassa which is well established as a model organism. The molecular mechanism in Neurospora and in other organisms (including humans) indicates that the biological clock consists of a network of gene-regulating processes. In Neurospora it is assumed that the frequency gen plays an important role in the creation of the central main oscillator. Christensen has now shown that the gene-regulatory processes which control the rhythm in Neurospora are in addition dependent on environmental factors such as the availability of nutrients.

When Neurospora uses nitrate (NO₃^-) as a nitrogen source (in stead of ammonia (NH₄⁺) which is usually used in studies) the rhythm is independent of the frequency gene, while the genes responsible for regulation of assimilation and metabolism of nitrate (i.e. the gene nit-3, which encodes the enzyme, nitrate reductase) seem to replace the role which the gene frequency is assumed to have. “We are beginning to understand that circadian rhythms can be based on separate oscillators that can be up-regulated as a result of environmental conditions”, says Christensen. Mathematical modeling calculations which Melinda has done in connection with nitrogen regulation in Neurospora support her experimental observations and her understanding of the data.

Melinda K. Christensen grew up in Orange Park and is the daughter of Tom and Cathy Moe. She attended Orange Park High School and was in the first graduating class of Middleburg High School in 1982. She has a Masters in Engineering, with majors in Chemistry, Offshore environmental technology and Biotechnology from the University of Stavanger, Norway. Her PhD work was done at the University of Stavanger and Dartmouth Medical School. She currently resides in Stavanger, Norway.