Aspartate aminotransferase in Leptosphaeria michotii has previously been shown to have an activity rhythm in constant conditions. The enzyme is present as two isofonns whose levels were quantified along the activity rhythm by ELISA, using specific polyclonal antibodies raised against polypeptides purified to a state of apparent homogeneity. The time-course of the level of the cytosolic isoform remained unchanged along the experiment. On the contrary, the cyclic variations in amount and transaminase activity (using cysteine sulfinate as substrate) of the mitochondrial isofonn gave rise to the aspartate aminotransferase activity rhythm of the fungus. The mRNA levels of the two isofonns, as determined by in vitro heterologous translation, remained monotonous along the daily cycle. These results and the sensitivity of the rhythm towards protein synthesis inhibitors are consistent with the hypothesis that the aspartate aminotransferase activity rhythm in this species is caused by some mechanism controlling the efficiency of translation of mitochondrial isoform mRNA.
