Physarum polycephalum is a lower eukaryote belonging to the amoebozoa group of organisms that forms macroscopic, multinucleate plasmodial cells during its developmental cycle. Plasmodia can exit proliferative growth and differentiate by forming fruiting bodies containing mononucleate, haploid spores. This process, called sporulation, is controlled by starvation and visible light. To genetically dissect the regulatory control of the commitment to sporulation, we have isolated plasmodial mutants that are altered in the photocontrol of sporulation in a phenotypic screen of N-ethyl-N-nitrosourea (ENU) mutagenized cells. Several non-sporulating mutants were analyzed by measuring the light-induced change in the expression pattern of a set of 35 genes using GeXP multiplex reverse transcription-polymerase chain reaction with RNA isolated from individual plasmodial cells. Mutants showed altered patterns of differentially regulated genes in response to light stimulation. Some genes clearly displayed pairwise correlation in terms of their expression level as measured in individual plasmodial cells. The pattern of pairwise correlation differed in various mutants, suggesting that different upstream regulators were disabled in the different mutants. We propose that patterns of pairwise correlation in gene expression might be useful to infer the underlying gene regulatory network.
Immunohistochemical demonstration of overexpression of the p53 protein indicates a mutational alteration of the gene. Our own investigations of 59 differentiated thyroid carcinomas revealed an overexpression in 15% of the tumors. A correlation to unfavourable tumor prognosis was found (stage I and II: 0/11 (0%); stage III: 4/26 (14%); stage IV: 5/22 (23%)). For screening of one out of more than 300 possible mutations temperature gradient gel electrophoresis was employed. Analysis of the highly-conserved regions of the p53 gene (exon 5 to 8) could demonstrate a mutation in only 1 out of 31 differentiated thyroid carcinomas. The question arises whether accumulation of the protein is due to a mutational event or rather other molecular mechanisms.
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