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

Differential messenger RNA gradients in the unicellular alga Acetabularia acetabulum. Role of the cytoskeleton

The unicellular green alga Acetabularia acetabulum has proven itself to be a superior model for studies of morphogenesis because of its large size and distinctive polar morphology. The giant cell forms an elongated tube (a stalk of up to 60 mm in length), which at its apical pole makes whorls of hairs, followed by one whorl of gametophores in the shape of a cap. At its basal pole, the cell extends into a rhizoid wherein the single nucleus is positioned. In this study, we have determined the level of specific messenger RNAs in the apical, middle, and basal regions using reverse transcriptase-PCR methodology. Four mRNA classes were distinguished: those that were uniformly distributed (small subunit of Rubisco, actin-1, ADP-glucose, centrin, and alpha- and beta-tubulin), those that expressed apical/basal (calmodulin-4) or basal/apical gradients (calmodulin-2 and a Ran-G protein), and those with development-specific patterns of distribution (mitogen-activated protein kinase, actin-2, and UDP-glucose-epimerase). Restoration of the apical/basal calmodulin-4 mRNA gradient after amputation of the apical region of the cell requires the nucleus and was abolished by cytochalasin D. Accumulation of actin-1 mRNA in the vicinity of the wound set by the amputation needs, likewise, the presence of the nucleus and was also inhibited by cytochalasin. This suggests that actin microfilaments of the cytoskeleton are involved in directed transport and/or anchoring of these mRNAs.

A close temporal and spatial correlation between cell growth, cell wall synthesis and the activity of enzymes of mannan synthesis in Acetabularia mediterranea

The synthesis of cell wall mannan and the activities of guanosine-diphosphate-mannose-pyrophosphorylase (EC2.7.7.13) and mannan synthetase were studied during the development of nucleate and enucleated cells of the alga Acetabularia mediterranea. The activities of both enzymes are relatively high as long as the cells grow and synthesize mannans. With termination of growth and mannan synthesis, the activities of both enzymes, but especially of mannan synthetase, drop to a low value. Furthermore, the activities of both enzymes are distributed in the cell along an apical-basal gradient. High activities are present in the apical regions of the cell where growth and mannan synthesis mainly occur, whereas in the basal region, growth, mannan synthesis and the activity of the two enzymes are slight. Since the in vitro activity of GDP-Man-pyr is at least 100 times higher than that of mannan synthetase, it was concluded that mannan synthetase activity is the limiting factor in mannan synthesis. This conclusion is supported by the determined pool sizes of Fru 6-P, Man 6-P, Man 1-P and GDP-Man during the development of the cells. The control of mannan synthesis and with it cell wall formation and growth through the regulation of mannan synthetase activity is discussed.

Model for the positional differentiation of the cap in Acetabularia

A late stage during the biological cycle of the unicellular alga Acetabularia is the differentiation of a cap at the apical end of the stalk. A minimal model of the spatio-temporal regulation of this event is proposed on the basis of biological data available and current hypotheses. This involves the interaction between a diffusing inhibitor specific to the translation of cap mRNAs and a graded distribution of these messengers. The model accounts for delayed protein synthesis which occurs preferably at the apex and is likely to initiate the formation of the cap. The biological and theoretical implications are discussed.

[The activity of phosphoglucose-isomerase and phosphoglucomutase during morphogenesis of nucleate and anucleate cells of Acetabularia]

The activity of two enzymes of the UDP-glucose and UDP-galactose biosynthetic pathway-phosphoglucose-isomerase and phosphoglucomutase-have been followed during morphogenesis of the unicellular green alga Acetabularia mediterranea. We have found that the increase in enzyme activities during morphogenesis represents de novo synthesis. The kinetic constants (K m, K i) of phosphoglucose-isomerase are similar to those found for the enzyme from rabbit muscle; mannose-6-phosphate was found to be a competitive inhibitor.The increase in activity of phosphoglucose-isomerase parallels the linear increase in total protein, neither showing changes with cap formation. In contrast, the activity of phosphoglucomutase is enhanced markedly when cap formation starts.Whether the nucleus is present or absent does not affect the above mentioned properties of the two enzymes. From this fact we conclude that the enzyme syntheses are regulated by the cytoplasm, probably at the level of translation. Contributions of DNA from chloroplasts and mitochondria can be excluded because in anucleate cells, which have an already established gradient of m-RNA, actinomycin D (10 μg/ml) has no effect on the synthesis of these enzymes.There are also differences in the spatial distribution of the two enzymes. We were able to show that both enzymes are synthesized unequally in different regions of the cells.