Changes in the polyadenylated messenger RNA population during development of Dictyostelium discoideum

mRNA stabilization controls the expression of a class of developmentally regulated genes in Dictyostelium discoideum

During the development of Dictyostelium discoideum, several thousand new mRNA species appear in the cytoplasm after the cells have formed stable aggregates. Here we show that six of these late mRNAs, corresponding to six clones randomly chosen from a genomic library, are synthesized from the very beginning of development at a rate comparable to that observed late in development but that transcripts do not accumulate until after aggregation. The early- and late-synthesized mRNAs are identical in size and compete with each other for hybridization to the genomic clones. The early-synthesized mRNAs do not accumulate in the cytoplasm in the preaggregation stage because they are very unstable. Their stability, estimated from the kinetics of incorporation during continuous labeling with (32)P, increases by perhaps an order of magnitude in the postaggregation stage. We conclude that mRNA stabilization is the major controlling factor of the expression of these genes.

Cell-type-specific genes expressed late in Dictyostelium development show markedly different responses to 3'5' cyclic AMP

The maintenance of late gene expression by 3'5' cyclic AMP was re-examined using several newly isolated cell-type-specific genes. Expression of all the prespore-enriched genes ceased immediately upon disaggregation of developing cells and pre-existing mRNA was rapidly degraded. For most genes, cAMP had little or no effect either alone or in combination with conditioned medium factors. The expression of the non-cell-type-specific genes 7E and 2C also ceased upon cell disaggregation but cAMP triggered a full re-induction of expression although the timing of the response differed markedly between these two genes. In contrast to earlier interpretations, these data argue that for none of these late prespore genes is cAMP alone sufficient for the maintenance of expression. The responses of the two prestalk mRNAs examined were gene-specific. Prestalk 5D mRNA decayed slowly upon disaggregation and was partially stabilized by cAMP whereas prestalk 5G mRNA increased upon disaggregation and was inhibited by cAMP.

Developmental regulation of cell-type-enriched mRNAs in Dictyostelium discoideum

We describe sixteen new families of cDNA clones representing mRNAs that are expressed preferentially in either prespore or prestalk cells during development of Dictyostelium discoideum and two new mRNAs that are expressed in a non-cell-type-specific manner. None of the prespore-enriched mRNAs are detectable in Dictyostelium cells until 13-15 h of development but then they increase dramatically and peak at 18-22 h. Upon dissociation of developing aggregates, all these mRNAs rapidly decay to low levels. In marked contrast to data presented for prespore genes by other workers, cyclic AMP either has no effect on the mRNA levels in dissociated cells or is only weakly effective in restoring normal expression. A prestalk-enriched mRNA examined, 5G mRNA, is similarly expressed late in development but is also expressed in vegetative cells. The level of 5G mRNA is only moderately affected by cell disaggregation.

Two cyclic AMP-regulated genes from Dictyostelium discoideum encode homologous proteins

Expression of the 7E and 2C genes late in Dictyostelium development ceases upon cell disaggregation but, in contrast to many other genes we have studied, expression is fully restored by exogenous cAMP (A. J. Richards et al., submitted). The 7E and 2C genes encode polypeptides of similar size (9220 and 10573 Daltons, respectively), each of which contains an unusually high proportion of serine plus glycine residues (41% and 59%, respectively). Each protein possesses a relatively serine-rich N-terminus and glycine-rich C-terminus and contains the conserved sequence S(X)SSS(X2)SS(X)SS(X2)SFGS. These data suggest that genes 7E and 2C may have arisen by duplication of a common ancestor. Computer analysis indicates that both gene products are probably intracellular structural proteins that form extended coil structures.

Developmental control of the expression of the dihydroorotate dehydrogenase and UMP synthase genes in Dictyostelium discoideum

Developmental variations in the expression of two genes of the de novo pyrimidine biosynthetic pathway have been examined in Dictyostelium discoideum. One gene, DdPYR4, encodes the dihydroorotate dehydrogenase (EC 1.3.3.1); the other, DdPYR5-6, encodes the UMP synthase which in D. discoideum is a bifunctional enzyme harboring both the orotate phosphoribosyl transferase activity (EC 2.4.2.10) and the OMP decarboxylase activity (EC 4.1.1.23). The relative amount of mRNA for both genes has been estimated by hybridization with the previously cloned DNAs and compared with the amount of actin mRNA. The level of both mRNAs is dramatically reduced after 4 h of development and remains at a low level later in development. In contrast to these variations, the specific activity of the enzymes encoded by these genes during development is similar to that measured during exponential growth. These results lead us to propose that DdPYR4 and DdPYR5-6 genes encode for relatively stable proteins and that their synthesis is reduced to maintain a constant level of enzymes in non-growing cells. This mode of regulation could apply to a large number of housekeeping genes.

Genetic programming of development: a model

Genetic programming of the developmental processes in multicellular organisms is proposed to be so intricate and vitally important that a large set of genes is dedicated solely to this end. It is further proposed that this set can be compartmentalized into subsets on the basis of the changes in gene activities that occur during ontogenesis, and that the genes in each subset transiently control the epigenetic activities of a small group of cells. Automatic subset activation is achieved by the product of a gene in each subset that transfers activity specifically to the subset next in the developmental sequence. This device can generate a unidirectional series of activations that cascade hierarchically through development like toppling dominoes. The model provides a basis for developmental phenomena, such as pattern formation, morphogenesis, and regeneration, and it makes testable predictions at the molecular level.

Genes selectively expressed in proliferating Dictyostelium amoebae

Few eukaryotic genes are expressed only during cell growth and division. We found that the slime mold Dictyostelium discoideum is unusual in that it expresses many genes only during proliferation. Thirty-two percent (304/950) of the sequences in a cDNA library made from vegetative mRNA were homologous to RNAs that are present at high levels during growth but at low or undetectable levels during differentiation when no cell growth occurs. In vitro translation assays confirmed that one-third of the vegetative cell mRNAs decreased in steady-state levels during differentiation. These vegetative cell-specific transcripts identified a diverse coordinately regulated class of genes: (i) 9 of the 10 cDNAs tested hybridized to unique small transcripts ranging from 400 to 620 bases long; (ii) the sequences showed various degrees of homology to related species; (iii) transcript levels synchronously fell by a factor of greater than 20 during development and synchronously increased during germination. This class of genes may play important roles in normal cell proliferation.

Major changes in gene expression occur during at least four stages of development of Dictyostelium discoideum

The spectrum of proteins synthesized at different stages of development of the cellular slime mold Dictyostelium discoideum was analyzed by two-dimensional (2D) gel electrophoresis. Of the approximately 400 proteins detected by this method 189 show changes in their relative rate of synthesis. Most of these changes occur during four distinct stages of development: commencement of development immediately following removal of nutrients (early interphase), early aggregation, late aggregation, and culmination. During commencement the synthesis of 19 proteins begins, the relative rate of synthesis of 21 other proteins increases, and 16 proteins show a rapid decrease in their synthetic rate. During early aggregation the largest change occurs in the spectrum of proteins being synthesized. Specifically, the synthesis of 29 new proteins begins and an increase occurs in the relative synthetic rate of 43 others. During late aggregation, when tight cell-cell contacts form, a reduction takes place in the synthetic rate of most of these induced proteins in addition to the synthesis of 12 new proteins. At least two of these induced proteins are synthesized exclusively in prespore and eventually spore cells. Finally, during culmination, 23 new proteins begin to be synthesized and the synthetic rate of 12 other proteins increases. Five of the 23 newly synthesized proteins appear to be stalk-cell specific. In general, synthesis of spore-cell specific proteins begins just following the formation of tight aggregates while stalk-cell specific proteins are induced during culmination. The relative amounts of mRNAs coding for most of the early developmentally regulated proteins have been estimated by their translation in rabbit reticulocyte lysates and subsequent analysis of protein products by 2D gel electrophoresis. For most of those proteins whose rate of synthesis increases in vivo following starvation there is a parallel increase in the cellular level of the functional mRNAs encoding them. This suggests that the genes coding for these mRNAs may be under transcriptional control. In contrast, the mRNAs coding for most of the proteins whose synthetic rate decreases early in development are under translational control and persist in the cell in an inactive state.

Expression of a spore-specific gene in Dictyostelium discoideum

The expression of a previously cloned Dictyostelium discoideum spore-specific gene (Julien et al., EMBO J. 1, 1089-1093 (1982)) was investigated in wild type and mutant strains. In vitro translation of this spore-specific mRNA gave a protein of a molecular weight consistent with the mRNA size. Expressed at a low level during vegetative growth development and in stalk cells, the accumulation of this mRNA reached high values only in spore cells.

Changes in the polyadenylated messenger RNA population during differentiation of Vicia faba root cells

Using cDNA probes, the polysomal polyadenylated RNA populations of Vicia faba meristematic, elongating, and mature root cells were analysed and compared, with respect to complexity, abundance distribution, and sequence representation. These cells express 12 300, 15 800, and 15 200 sequences, respectively, of an average size of 1300 nucleotides, distributed in three frequency classes. Transition from the meristematic to the elongating stage is coordinated with the disappearance of 25-30% of the abundant RNA species (2000 copies per cell) and with the appearance of new transcripts corresponding to 3000-3500 genes expressed at an intermediate level (50-60 copies per cell) and belonging to the rare class (about 4 copies per cell). These new transcripts represent 12.5% of the mass of the mRNA during the elongating stage and are quantitatively modulated during the transition to the mature stage. Thus 80 to 85% of the polysomal polyadenylated RNA species expressed in elongating or mature cells are common to the three cell types.