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Transcription of the 117 gene and changes in its mRNA levels in Dictyostelium discoideum were studied by mRNA hybridization with a cDNA probe. In wild type cells (Ax-2), the expression is developmentally regulated during cell aggregation, while in the aggregateless mutant, Agip 45, 117 mRNA is not detectable during cell starvation. Low concentrations of cAMP, given in the form of extracellular pulses to induce the development of starved Agip 45 cells to aggregation competence, are able to induce the appearance of 117 mRNA. The induction seems to be via the cell surface cAMP receptor and by a mechanism which does not involve changes in intracellular cAMP. Interestingly, high concentrations of cAMP, which down-regulate the cell surface cAMP receptor, elicit a rapid decrease in the level of 117 mRNA in aggregation-competent cells. Nuclear run-off and pulse-chase experiments show that the high concentrations of cAMP selectively destabilize the mRNA for 117 antigen. This destabilization requires both de novo mRNA synthesis and protein synthesis since the addition of inhibitors of these processes eliminates the effects of cAMP on 117 mRNA. The data suggest that a cAMP-induced protein(s) may be involved in the destabilization of selective mRNAs.

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A monospecific polyclonal antiserum to the regulatory subunit (R) of the cAMP-dependent protein kinase of Blastocladiella emersonii has been developed by immunization with purified regulatory subunit. In Western blots, the antiserum displays high affinity and specificity for the intact R monomer of Mr = 58,000, as well as for its proteolytic products of Mr = 43,000 and Mr = 36,000, even though the antiserum has been raised against the Mr = 43,000 fragment. Western blots of cell extracts prepared at different times during the life cycle of the fungus indicate that the increase in cAMP-binding activity occurring during sporulation, as well as its decrease during germination, are associated with the accumulation of the regulatory subunit during sporulation and its disappearance during germination, respectively. Pulse labeling with [35S]methionine and immunoprecipitation indicate that the accumulation of R is due to its increased synthesis during sporulation. Two-dimensional gel electrophoresis of affinity purified cell extracts obtained after [35S]methionine pulse labeling during sporulation confirms de novo synthesis of R during this stage and furthermore shows that the protein is rapidly phosphorylated after its synthesis. In vitro translation studies using RNA isolated from different stages of the life cycle followed by immunoprecipitation have shown that the time course of expression of the mRNA coding for the regulatory subunit parallels the rate of its synthesis in vivo.

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Actin and alpha and beta-tubulin have been identified in Blastocladiella emersonii by two-dimensional gel electrophoresis and Western blotting. The kinetics of synthesis of these proteins were compared by pulse-labeling experiments with [35S]methionine and with the accumulation of their corresponding mRNAs, translated in a cell-free system. Large increases occur in the rates of actin and alpha- and beta-tubulin biosynthesis during sporulation and there is an accumulation of the corresponding mRNAs. In parallel to the increased synthesis, these cytoskeletal proteins accumulate during the late stage of sporulation.

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Extracts of aggregation-competent cells of Dictyostelium discoideum have an S6 protein kinase activity which is inhibited in the presence of the inhibitor of the cAMP-dependent protein kinase. The phosphorylation of S6 is rapid, and decays rapidly. The S6 kinase activity is detectable in the 150,000g supernatant only in the presence of phosphatase inhibitors known for preserving the S6 kinase in other systems, indicating that the activated form of the enzyme is phosphorylated by the cAMP-dependent protein kinase. S6 kinase elutes as a peak from DEAE-Sephacel at 100 mM NaC1, with an activity that is cAMP-dependent.

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Changes in phosphorylation of ribosomal protein S6 during heat shock, induction of thermotolerance and recovery from heat shock at different stages of Blastocladiella emersonii development were investigated. Independently of the initial state of S6 phosphorylation (maximal or intermediate), a rapid and complete dephosphorylation of S6 is induced by heat shock and S6 remains unphosphorylated during the acquired thermotolerance. During recovery from heat shock rephosphorylation of S6 occurs always to the levels characteristic of that particular stage, coincidently with the turn off of heat shock protein synthesis.

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Ribosomal proteins of the aquatic fungus Blastocladiella emersonii were isolated and characterized on four different two-dimensional polyacrylamide gel electrophoresis systems. 40S and 60S ribosomal subunit proteins from zoospores were identified. The position of every protein was determined in each electrophoretic system using the "four-corners" method (Madjar et al., Molecular and General Genetics, 171: 121-134, 1979). Thirty-two and 39 proteins were identified in the 40S and 60S ribosomal subunits, respectively. The molecular weights of individual proteins in the 40S subunit ranged from 10 000 to 37 000, with a number-average molecular weight of 20 000. The molecular weight range for the 60S subunit was 13 000-51 000 with a number-average molecular weight of 21 000. Proteins from ribosomes of different cell types were compared and found to be qualitatively indistinguishable. The only consistent difference in the patterns of proteins was in the S6 protein of the 40S subunit, which is the major phosphoprotein of Blastocladiella ribosomes.

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There was reduction in the levels of some liver endoplasmic reticulum proteins from streptozotocin-treated rats compared to those of normal rats as detected by two-dimensional polyacrylamide gel electrophoresis. When insulin was administered 24 h before streptozotocin, the protein patterns were the same as that obtained for normal rats. The comparison of the rate of synthesis of some endoplasmic proteins by streptozotocin-treated rats with that of streptozotocin-treated rats which received insulin 4 h prior to the experiment by the double-labelling technique indicated that insulin increased the rate of synthesis of some liver endoplasmic reticulum proteins. The rates of synthesis of some liver ribosomal proteins from streptozotocin-treated rats were increased and others were decreased when the animals were compared with streptozotocin-treated rats pretreated with insulin 4 h earlier. These results suggest that the generalized decrease of the rate of protein synthesis which has been reported in diabetes is due to a decrease of the rate of synthesis of some ribosomal proteins that may act in chain initiation or have a regulatory function.

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