The utilization of mixtures of glucose and sucrose at nonlimiting concentrations was studied in batch cultures of two common thermophilic fungi, Thermomyces lanuginosus and Penicilium duponti. The sucrose-utilizing enzymes (sucrose permease and invertase) in both fungi were inducible. Both sugars were used concurrently, regardless of their relative proportion in the mixture. At the optimal growth temperature (50C), T. lanuginosus utilized sucrose earlier than it did glucose, but at a suboptimal growth temperature (30°C) the two sugars were utilized at nearly comparable rates. The coutilization of the two sugars was most likely possible because (i) invertase was insensitive to catabolite repression by glucose, (ii) the activity and affinity of the glucose transport system were lowered when sucrose was included in the growth medium, and (iii) the activity of the glucose uptake system was also subject to repression by high concentrations of glucose itself. The concurrent utilization of the available carbon sources by thermophilic fungi might be an adaptive strategy for opportunistic growth in nature under conditions of low nutrient availability and thermal fluctuations in the environment.Since Monod (21) described the phenomenon of diauxy in bacteria, it has become generally accepted that, when supplied with a mixture of two carbohydrates, the microorganisms utilize one carbon source at a time; for example, glucose is utilized before lactose is. There has been virtually no study of mixed substrate utilization in fungi, although this situation is expected to occur often in nature. We investigated the ability of thermophilic fungi to utilize mixed carbon sources. Thermophilic fungi are commonly found in compost and soil (17). Their optimum growth and metabolic activity occur between 45 and 55°C (5, 23). It is believed that they are capable of opportunistic development in the surface layers of soil when the soil temperatures become favorable because of solar heating (13). We considered the possibility that one of the adaptive strategies of thermophilic fungi growth in the thermally fluctuating (25) and nutritionally poor environment of soil (9) might be their ability to utilize the available carbon sources rapidly and simultaneously. We report here on the ability of two unrelated species of thermophilic fungi to concurrently utilize glucose and sucrose when provided in combination under substrate-sufficient concentrations and on some of the mechanisms that underlie this behavior. MATERIALS AND METHODSOrganisms. Thermomyces lanuginosus Tsiklinsky RM-B (ATCC 44008) was used in all experiments. In some experiments, Penicillium duponti Griffon and Maublanc RM-D was also used. Both fungi were taken from our laboratory collection of thermophilic fungi.Culture conditions. Mycelial inocula were prepared by incubating spores in a glucose-asparagine medium on a gyratory shaker (240 rpm) at 50°C, as described by Maheshwari et al. (16). For P. duponti, the medium was supplemented with 0.1% (wt/vol) yeast extract (Difco Laboratori...

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