The structure, function and distribution of the cytoskeletal protein actin will be studied at the biochemical and light and electron microscopical levels in the fission yeast Schizosaccharomyces pombe and in the budding yeast Saccharomyces cerevisiae. These organisms have been chosen for analysis because they are simple unicellular eukaryotes with well established genetics and are easily manipulated by sophisticated techniques of modern biochemistry and cell biology. The proposed study centers on three areas of investigation: firstly, the characterization of actin both structurally and functionally; secondly, the light and electron microscopic morphology of the actin cytoskeleton in rapidly frozen cells; and thirdly, the analysis of actin regulation by posttranslational modification. These studies in the cell biology and biochemistry of the actin cytoskeleton will establish a defined system for the molecular biological analysis of the actin cytoskeleton and the regulations of actin at the level of the protein itself. As the molecular biology of the single actin genes in both yeasts is well documented, future studies of mutant actins created by site- specific mutagenesis and returned to the yeast genome can allow for the ultrastructural and functional analysis of actin in vivo. As actin appears to be ubiquitous amongst eukaryotes and is differentially expressed during development in mammalian organisms, these studies will ultimately contribute to the understanding of cellular differentiation and functioning of this ubiquitous cytoskeletal protein.