is a hyperthermophilic bacterium whose phylogenetic lineage includes the most primitive of the bacterial heterotrophs. It is not known whether exhibits preferences for growth substrates or regulates the synthesis of degradative enzymes. We have found that exhibits diauxic growth in medium containing 300 μM glucose and 1 mM lactose. We measured the activity of β-galactosidase and β-glucosidase in extracts prepared from cells grown on defined media and found that cells grown on 0.5% lactose, galactose or cellobiose contained β-galactosidase specific activities of 1.19, 1.78 and 1.34 U (mg protein), respectively. Cells grown on 0.5% glucose, maltose, fructose, sucrose, xylose, ribose or starch had no measurable β-galactosidase activity. β-Glucosidase activity was found only in cells grown on cellobiose. Cells grown on the combination of 0.5% lactose or galactose and 0.05% glucose had no detectable β-galactosidase activity, whereas up to 0.5% glucose did not prevent expression of β-galactosidase or β-glucosidase activity in cells induced with 0.5% cellobiose. These activities are catalysed by separate enzymes as determined by resolution of their activities on 6% native polyacrylamide gels. Therefore, only β-galactosidase synthesis induced by lactose is subject to catabolite repression. To determine the mechanism of catabolite repression, the levels of cAMP were measured in cells grown on various defined media using an enzyme-immunoassay. The cAMP levels ranged from 44 to 280 fmol (mg protein) irrespective of the carbon source used. By comparison, grown on lactose contained 5.1 pmol (mg protein). Like Gram-positive bacteria, displays a cAMP-independent mechanism for catabolite repression and this may represent the more ancient mode of regulation.


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