Evidence is presented to suggest that several early events of zoospore germination in the water mold, Blastocladiella emersonii, are not dependent upon concomitant protein synthesis. The events involve abrupt, dramatic changes in cell architecture. On account of this evidence, as well as evidence available from other sources, we question whether differential protein synthesis provides an exclusive and sufficient mechanism for phenotypic change (i.e., cell differentiation). Rather, we argue that mechanisms for bringing about structural alterations in the preformed machinery of the cell should also be given attention.One common thread runs through virtually all current thinking regarding the mechanism(s) of cellular differentiationnamely, that cells change phenotype (i.e., differentiate) as a result of differential protein synthesis. Our intention in this paper is certainly not to discredit the usefulness of this hypothesis, but rather to call into question whether this form of phenotypic control provides an exclusive and sufficient mechanism for cellular differentiation.The organism under study is the water mold Blastocladiella emersonii. We have focused attention particularly on the sequence of cellular changes involved in zoospore germination, primarily because this sequence is characterized by dramatic, abrupt changes in cell structure, and because zoospore populations can be induced to germinate rapidly and semisynchronously (1). A sequence of four morphologically distinct cell types can be unambiguously resolved (see Fig. 1 and refs.
Analysis of protein degradation during the life cycle of Blastocladiella emersonii showed that (i) protein degradation is especially high during two phases of differentiation (sporulation, 12%o/h and germination, 5%o/h) in contrast with a much smaller degradation rate in the other phases (growth and zoospores, less than 10o/hr); (ii) protein degradation during germination in growth medium, as well as most of the germination process, is quantitatively unaffected by cycloheximide; (iii) a caseinolytic protease (pH optimum 5.5, apparent molecular weight 55,000 to 60,000) is present in extracts of zoospores and germinating cells; (iv) this protease activity is very low (perhaps absent) in extracts of late growth phase cells, but reappears during induced sporulation; (v) a different class of caseinolytic protease activity (pH optima 7 and 10; apparent molecular weight 25,000 to 30,000) is found in cellular extracts of late growth phase and early phases of sporulation; (vi) the latter class of enzyme activity is released into the medium during later phases of sporulation and is replaced in the cells by the former class. Speculations as to the roles of protein degradation in cell differentiation are discussed.
De novo construction of a chitinous cell wall accompanies Blastocladiella emersonii zoospore germination. At least an order of magnitude increase in total hexosamine occurs during germination. This increase is into polymer (chitin) and occurs on schedule in the presence of cycloheximide. Uridine-5'-diphospho-N-acetylglucosamine (UDPGlcNAc), both the end product of hexosamine biosynthesis and a substrate for chitin biosynthesis, is a potent inhibitor of the activity of the first pathway-specific enzyme of hexosamine biosynthesis in zoospore extracts. Certain uridine nucleotides, not perceptibly influencing the activity of the first enzyme per se, counteract the inhibitory effects of UDPGIcNAc. The concentration of UDPGlcNAc in the zoospore is sufficient to act as an inhibitor of the enzyme, but the amount of UDPGlcNAc is insufficient, by over an order of magnitude, to account for the chitin synthesized during germination. Both the production of UDPGlcNAc and its utilization for chitin synthesis appear to be post-translationally regulated in zoospores and during zoospore germination.
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