Protein synthesis in the absence of the nucleus

A cytological study of the centrifuged whole, half, and quarter eggs of the sea urchin, Arbacia punctulata

While the ooplasmic components of centrifuged eggs of Arbacia punctulata do not stratify in homogeneous layers, we have obtained the following strata beginning with the centripetal end: lipid droplets, pronucleus, clear zone, mitochondria, yolk, and pigment. Whereas mitochondria may be found mingled with yolk bodies, we have never observed lipid droplets nor pigment bodies among any of the other inclusions. The so-called clear zone contains a heterogeneous population of inclusions: annulate lamellae, heavy bodies, Golgi complexes, and rod-containing vacuoles. The peripheral cortical granules of immature (germinal vesicle stage) and of mature eggs are not dislodged from the cortical ooplasm with the centrifugal force utilized. When the eggs are treated with urethane, prior to centrifugation, the cortical granules of mature eggs abandon their peripheral position. Further centrifugation of the initially stratified eggs produces nucleated and nonnucleated halves and the centrifugation of the halves results in quarters. The cytology of the halves and quarters is discussed. The halves and quarters have been activated with either sperm or hypertonic sea water. With the exception of the nucleated halves, we were unable to obtain plutei larvae from the other fractions (red halves and quarters). We believe that the lack of development of the various fragments is a function of the balance of particular inclusions necessary for differentiation.

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

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.

[The effect of inhibitors of RNA- and protein synthesis on the chlorophyll content of nucleate and anucleate cells of Acetabularia]

The chlorophyll content in anucleate cells of the green alga Acetabularia mediterranea increases during about four weeks after enucleation. This corresponds exactly to the peroid of total protein synthesis and synthesis of the enzyme UDPG-pyrophosphorylase, which is coded by nuclear DNA. When nucleate posterior pieces of the stalk are treated with actinomycin (10 μg/ml), the increase in chlorophyll content is blocked completely after a lag period. Morphogenesis and synthesis of the UDPG-pyrophosphorylase are strongly inhibited also. As is well known, anucleate cells of Acetabularia contain a storage of long lived nucleus dependent messenger-RNA within the tip region of the stalk. Therefore actinomycin has only a little influence on stalk and cap formation and no influence on the synthesis of the UDPG-pyrophosphorylase in anucleate cells. On the other hand the antibiotic strongly inhibits the increase in chlorophyll content in such pieces.Chloramphenicol (100 μg/ml) completely blocks the increase in chlorophyll content in both nucleate and anucleate cells but inhibits the synthesis of the UDPG-pyrophosphorylase only slightly in the beginning. Inhibition which sets in later is very likely a secondary effect. Actidion (0.25 μg/ml) and puromycin (30 μg/ml) strongly inhibit the increase in chlorophyll content and UDPG-pyrophosphorylase synthesis in both nucleate and anucleate cells. This inhibition is partly reversible. It is concluded from these results that the chlorophyll content of the cells is controlled not only by nuclear genes but also by the DNA of the plastids and that protein synthesis is necessary for chlorophyll formation or stabilization.Some possibilities of how chloroplast DNA may control chlorophyll formation and stability are discussed.

Control of specific gene expression examined in synchronized mammalian cells

The control of synthesis of the steroid-inducible enzyme, tyrosine aminotransferase (TAT), has been studied in synchronized cultures of HTC cells, an established line of rat hepatoma cells. During the mitotic and early G1 periods of the generation cycle of cells previously exposed to inducer, TAT synthesis is maximal in the absence of inducer, i.e., synthesis is constitutive. (By contrast, in random cells maximal TAT synthesis is always dependent upon the presence of the inducer.) After the third hour of G1 "G1[3]"), TAT synthesis is inhibited by a specific posttranscriptional repressor, the formation of which requires RNA synthesis. We speculate that this repressor is antagonized by the steroid inducer. The degradation of TAT during mitosis and G1 is constant and not affected by cycloheximide or actinomycin D in our experiments.