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A potato particulate preparation, which is constituted mainly by proplastids, was capable of glucose transfer from micromolar concentrations of uridine diphosphate ['4C]glucose into an endogenous protein molecule to form a 14C-labelled glucoprotein. This glucoprotein had short radioactive glucan chains and could act as acceptor for the synthesis of larger radioactive glucan chains from labelled uridine diphosphate glucose, either at millimolar concentrations or in the presence of glucose 1 -phosphate or adenosine diphosphate glucose at micromolar concentrations. This last step required MnZ+ or M 2 + ions.The enzyme responsible for the formation of the glucoproteic acceptor was named uridine diphosphate glucose-protein transglucosylase I and its properties were studied.The uridine diphosphate glucose-protein transglucosylase activity, that was detected at micromolar uridine diphosphate glucose concentrations in the presence of glucose l-phosphate, could be distinguished from uridine diphosphate glucose -protein transglucosylase I by the following criteria: The glucose l-phosphate dependent activity required MnZ + or MgZ+ ions, was sensitive to temperature and formed radioactive glucan chains totally hydrolyzed by /&amylase. Conversely, uridine diphosphate glucose-protein transglucosylase I was active in the absence of Mn" or M g + ions, was resistent to temperature and formed radioactive glucan chains scarcely attacked by fl-amylase.Sequential analysis of the glucose incorporation from uridine diphosphate [3H]glucose and uridine diphosphate ['4C]glucose into the glucoprotein and sodium dodecylsulphate -acrylamide gel electrophoresis of the labelled glucoproteins formed in the presence and in the absence of glucose l-phosphate were carried out.On the basis of the data obtained a simple scheme of reactions is proposed to explain the involvement of the uridine diphosphate glucose -protein transglucosylase I and glucose 1-phosphate dependent activities in the synthesis of a-1,4 glucan chains of the starch type.Starch synthetase plays a main role in the formation of the complex structure of starch granules in plant cells. This enzyme, which is capable of synthesizing a-1,4 glucosidic chains, can be found in a particulate form, bound to the starch granule [1,2] and in a soluble state [3 -71. The specificity of the two forms of starch synthetase appears to be different. While reserve starch granules of potato and cereal grains can use ADP-glucose, UDP-glucose and other sugar nucleotides as glucosyl donors, the soluble enzymes transfer glucose specifically from ADP-glucose [2]. The latter specific requirement is also reported for certain starch granules [S, 91. These differences suggest that the particulate and soluble starch synthetases are different enzymes and the loss of the capability to use UDP-glucose, which is observed in starch granules after grinding and urea treatment [lo] supports this hypothesis.With regard to the initiation of starch biosynthesis the question remains as to the origin of primer in vivo. Star...

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