Absence of Diauxie during Simultaneous Utilization of Glucose and Xylose by<i>Sulfolobus acidocaldarius</i>

ABSTRACT Sulfolobus acidocaldarius utilizes glucose and xylose as sole carbon sources, but its ability to metabolize these sugars simultaneously is not known. We report the absence of diauxie during growth of S. acidocaldarius on glucose and xylose as co-carbon sources. The presence of glucose did not repress xylose utilization. The organism utilized a mixture of 1 g/liter of each sugar simultaneously with a specific growth rate of 0.079 h −1 and showed no preference for the order in which it utilized each sugar. The organism grew faster on 2 g/liter xylose (0.074 h −1 ) as the sole carbon source than on an equal amount of glucose (0.022 h −1 ). When grown on a mixture of the two carbon sources, the growth rate of the organism increased from 0.052 h −1 to 0.085 h −1 as the ratio of xylose to glucose increased from 0.25 to 4. S. acidocaldarius appeared to utilize a mixture of glucose and xylose at a rate roughly proportional to their concentrations in the medium, resulting in complete utilization of both sugars at about the same time. Gene expression in cells grown on xylose alone was very similar to that in cells grown on a mixture of xylose and glucose and substantially different from that in cells grown on glucose alone. The mechanism by which the organism utilized a mixture of sugars has yet to be elucidated.