Circadian rhythms in Neurospora crassa: oscillation in the level of an adenine nucleotide

Abstract

In a mutant strain (bd) of Neurospora, the biological clock is visibly expressed at the growing front of a mycelial mat by sequential periods of conidiating (spore forming) and non conidiating growth. The edges (8 mm) of the mycelium at different ages were sampled during a 31 hr period, and the adenine nucleotide levels were enzymatically assayed. In the edge region, the total adenosine 5' monophosphate (AMP) level showed an oscillation, with a minimum of 0.5 μmol/g (residual dry weight) and a maximum of 6.0 μmol/g. The total adenosine 5' triphosphate level and the total adenosine 5' diphosphate level showed no obvious oscillation. The oscillation in AMP content had many of the properties of a circadian rhythm. Its period was about 22 hr long, it was phase shifted by light, and it was damped out by continuous illumination. The oscillation in AMP level led to an oscillation in the overall cellular energy charge from 0.65 to 0.93. However, the energy charge calculation does not take into account any possible compartmentalization of AMP, and therefore must be interpreted cautiously. It is suggested that the underlying cause of the oscillation in AMP level could be a rhythmic, partial uncoupling of mitochondrial oxydative phosphorylation.