An early hypothesis put forward to explain thermophily in bacteria proposed that growth at high temperatures occurs because the denatured cellular proteins are quickly replaced by resynthesis. Two different groups compared the rate of protein breakdown in thermophilic and mesophilic fungi by feeding mycelia with radioactively labeled amino acid and monitoring the radioactivity in mycelial proteins after transferring mycelia to nonradioactive media (Miller et al., 1974; Rajasekaran and Maheshwari, 1990). Although both groups measured only protein breakdown, similar results are expected had protein turnover been measured. The rapid protein turnover hypothesis is not yet substantiated but since at their respective temperature optima both mesophilic and thermophilic fungi produce comparable amounts of mycelium, it is unlikely that this hypothesis is valid generally because energy expended in increased protein turnover in thermophilic fungi would be expected to affect their yield, which does not happen. However, as shall be noted in Section 10.5.2, certain enzymes could have a rapid turnover rate.
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