Ribokinase, the first
enzyme in
ribose catabolism, is altered in its expression in
ribose-utilizing
Novikoff hepatoma variants. 90% of the variants selected for their ability to use
D-ribose as a sole
carbon source show a change in
ribokinase activity. After non-selective growth, phenotypically unstable variants lose their altered expression and regain a parental form of expression of this
enzyme. In the variants,
ribokinase expression in non-inducible by the
carbon source and is unaffected by the growth phase of the cells. However,
ribokinase expression in both parental cells and variants is cell cycle-dependent. Parental
Novikoff hepatoma cells have three peaks of
ribokinase activity during the S, G2 and M phases. Variants are described which have high basal levels of
ribokinase and only a single peak of enzymatic activity during the S phase. In addition to changes in the level of
ribokinase, changes in the subcellular localization of the
enzyme have been found in some variants. While the change in the level of
ribokinase seems to be a property of the variant isolated, the change in subcellular location of
ribokinase can be readily achieved by culture conditions.