[Regulation of UDPG-pyrophosphorylase activity in Acetabularia : II. Unequal capacity for enzyme synthesis in different cell regions]

Abstract

It has already been shown that in the uninucleate green alga Acetabularia mediterranea, a close relationship in time exists between cap formation and the activity of the enzyme UDPG-pyrophosphorylase. The activity of the enzyme increases strongly when cap formation sets in. Similar changes in enzyme activity are observed in nucleate and anucleate cells. It is very likely that the increase in the activity of the UDPG-pyrophosphorylase during cap formation is a result of increased synthesis of the enzyme (Zetsche, 1968).In addition to the correlation in time between cap formation and enzyme activity, there exists a close correlation in space which is expressed in terms of a polar distribution of the enzyme. The activity of the enzyme is highest in the apical region of the cell, whereas toward the basal end it decreases sharply. This apico-basal gradient of enzyme activity is especially strong in cells with a cap. By means of several experiments it was excluded that the above results were the consequence of an unequal distribution of inhibitors or activators of enzyme activity or of a differential inactivation of the enzyme during homogenization. We therefore conclude that the differences in enzyme activity between various regions of the cell are based upon differences in the amount of the enzyme present within these regions.Further experiments clearly demonstrated that the polar distribution of the enzyme is the result of a preferential synthesis of the enzyme in the apical region of the stalk. The rate of enzyme synthesis is higher within the apical region than in the middle and basal region, and this is particularly evident at the time of onset of cap formation. We conclude that the messenger-RNA responsible for the synthesis of the UDPG-pyrophosphorylase is distributed in the form of an apico-basal concentration gradient. If we accept the existence of such a gradient, then this implies that the messenger-RNA which is synthesized within the nucleus in the rhizoidal end of the cell must be transported over a distance of 60 mm in a polar direction toward the tip of the stalk and must be accumulated there. Unequal distribution of messenger-RNA thus may represent an important factor in the morphogenesis of the cell. However it remains to be shown how such a distribution is brought about.