Denison 9722744 All cells respond to signals from the environment. This is true of micro-organisms, many of which are composed of only a single cell, and of individual cells of multicellular organisms such as humans. Single-celled organisms respond to environmental signals such as the availability of nutrients and increases in temperature, acidity or osmotic pressure of the environment. The response of micro-organisms to environmental signals is to alter cellular processes to ensure survival in the new conditions. For example, yeast cells respond to increases in environmental osmotic pressure by accumulating certain intracellular substances to balance the change in environmental osmotic pressure. These various responses of cells to environmental signals occur as a result of altered activity of genes whose products are required for these processes. As genes of these eukaryotic cells are located in the nucleus, the environmental signal must be transduced from the cell membrane, which separates the cell from the environment, to the nucleus. The signals are transduced by mechanisms called signal transduction pathways. Components of signal transduction pathways from micro-organisms are very similar to those of cells of multicellular organisms. Aspergillus nidulans is a filamentous fungus used as an experimental system to investigate a number of important questions in genetics and cell biology. Aspergillus possesses an ambient pH signal transduction pathway which ensures that enzymes which function extracellularly are produced under conditions of ambient pH at which they can function. Extracellular enzymes which function only at certain pH values include proteases and phosphatases which are required for nutrient acquisition by Aspergillus. The components of this ambient pH signal transduction pathway are the products of the pal genes, palA, B, C, F, H and I. At alkaline ambient pH, the pal pathway causes the activation of the pacC-encoded transcription factor which causes genes whose products fun ction in alkaline pH to be transcribed. The goal of this project is to determine how the pal pathway transduces the signal of alkaline ambient pH. The aims of the project are to complete the sequence of the palI gene, which will allow determination of the protein product that it encodes. Preliminary sequence information suggests that the palI-encoded protein, PalI, is a membrane protein and may therefore be the pH sensor in the signaling pathway, initiating the signal transduction in response to alkaline ambient pH. A strain of Aspergillus which is disrupted for the pall gene will also be created to determine the effect of a complete absence of PalI. Another goal of the project is to use immunofluorescence microscopy to determine if PalI is located in the cell membrane which will help determine if it is the ambient pH sensor. New mutations (suppressor mutations) in Aspergillus will also be isolated which overcome the lack of pH signal transduction in strains with mutations in palI. This is to isolate new genes whose products interact with PalI in transducing the signal of alkaline pH and new genes whose products function elsewhere in the signaling pathway.
