Construction of a UMP synthase expression cassette from Dictyostelium discoideum

Molecular basis of pregnancy-induced breast cancer protection

We have postulated that the lifetime protective effect of an early pregnancy against breast cancer is due to the complete differentiation of the mammary gland characterized by a specific genomic signature imprinted by the physiological process of pregnancy. In the present work, we show evidence that the breast tissue of postmenopausal parous women has had a shifting of stem cell 1 to stem cell 2 with a genomic signature different from similar structures derived from postmenopausal nulliparous women that have stem cell 1. Those genes that are significantly different are grouped in major categories on the basis of their putative functional significance. Among them are those gene transcripts related to immune surveillance, DNA repair, transcription, chromatin structure/activators/co-activators, growth factor and signal transduction pathway, transport and cell trafficking, cell proliferation, differentiation, cell adhesion, protein synthesis and cell metabolism. From these data, it was concluded that during pregnancy there are significant genomic changes that reflect profound alterations in the basic physiology of the mammary gland that explain the protective effect against carcinogenesis. The implication of this knowledge is that when the genomic signature of protection or refractoriness to carcinogenesis is acquired by the shifting of stem cell 1 to stem cell 2, the hormonal milieu induced by pregnancy or pregnancy-like conditions is no longer required. This is a novel concept that challenges the current knowledge that a chemopreventive agent needs to be given for a long period to suppress a metabolic pathway or abrogate the function of an organ.

Uridine 5'-Monophosphate Synthase Is Transcriptionally Regulated by Pyrimidine Levels in Nicotiana plumbaginifolia

To understand the regulation and expression of pyrimidine biosynthesis in plants, we have examined the effect of the metabolic inhibitor 5-fluoroorotic acid (FOA) on uridine-5'-monophosphate synthase (UMPSase) expression in cell cultures of Nicotiana plumbaginifolia. UMPSase is the rate-limiting step of pyrimidine biosynthesis in plants. Addition of FOA causes an up-regulation of UMPSase enzyme activity in cell cultures after a lag phase of several days. Western-blot analysis demonstrated that the up-regulation in enzyme activity was caused by increased expression of the UMPSase protein. Northern-blot analysis demonstrated a higher level of UMPSase mRNA in the FOA-induced tissues than in control tissues. Run-on transcriptional assays showed that the UMPSase gene was transcriptionally activated after FOA treatment. The mechanism of toxicity of FOA is through thymine starvation. We found that addition of thymine abrogated the FOA-mediated up-regulation of UMPSase. In addition, methotrexate and aminopterin, which affect thymine levels by inhibiting dihydrofolate reductase, also up-regulate UMPSase in N. plumbaginifolia cells.

Isolation and characterization of a novel cytokinesis-deficient mutant in Dictyostelium discoideum

Cytokinesis is a dramatic event in the life of any cell during which numerous mechanisms must coordinate the legitimate and complete mechanical separation into two daughter cells. We have used Dictyostelium discoideum as a model system to study this highly orchestrated event through genetic analysis. Transformants were generated using a method of insertional mutagenesis known as restriction enzyme-mediated integration (REMI) and subsequently screened for defects in cytokinesis. Mutants isolated in a similar screen suffered a disruption in the myosin II heavy chain gene, a protein known to be essential for cytokinesis and in a novel gene encoding a rho-like protein termed racE [Larochelle et al., 1996]. In the screen reported here we isolated a third type of mutant, called 10BH2, which also had a complete defect in cytokinesis. 10BH2 mutant cells are able to propagate on tissue culture plates by fragmenting into smaller cells by a process known as traction-mediated cytofission. However, when grown in suspension culture, 10BH2 cells fail to divide and become large and multinucleate. Phenotypic characterization of the mutant cells showed that other cytoskeletal functions are preserved. The distribution of myosin and actin is identical to wild type cells. The cells can chemotax, phagocytose, cap crosslinked receptors, and contract normally. However, the 10BH2 mutants are unable to complete the Dictyostelium developmental program beyond the finger stage. The mutant cells contain functional genes for myosin II heavy and light chains and the racE gene. Thus, based on these findings, we conclude that 10BH2 represents a novel cytokinesis-deficient mutant.

Functional complementation of yeast phosphofructokinase mutants by the non-allosteric enzyme from Dictyostelium discoideum

Phosphofructokinase (PFK) from yeast has been replaced by the non-allosteric isozyme from the slime mold Dictyostelium discoideum. This has been achieved by overexpression of the latter in a PFK-deficient strain of Saccharomyces cerevisiae under the control of the PFK2 promoter. Transformants complemented the glucose-negative growth phenotype exhibiting generation times on glucose-containing media similar to those of an untransformed strain being wild-type for yeast PFK genes. The PFK produced reacted with an antibody against D. discoideum PFK. It exhibited the same subunit size, quaternary structure and kinetic parameters than those of the wild-type enzyme, and was also devoid of specific regulatory properties.