Sporulation, spore germination, and yeast-hypha dimorphism in the ifiamentous fungus Mucor racemosus provide useful model systems to study cell development in eucaryotic cells. Three RAS genes (MRASI, MRAS2, and MRAS3) from M. racemosus have been cloned, and their nucleotide sequences have been determined. The predicted amino acid sequences and the sizes of the three MRAS proteins exhibit a high degree of similarity with other ras proteins, including that encoded by H-ras, which have been implicated in regulation of proliferation and development in eucaryotic cells by mediating signal transduction pathways. The MRAS proteins show conservation of functional domains proposed for ras proteins, including guanine nucleotide interaction domains, an effector domain, a binding epitope for neutralizing antibody Y13-259, and the COOH-terminal CAAX box, which is a site of thiocylation and membrane attachment. Amino acid sequences unique to each MRAS protein occur adjacent to the CAAX box, consistent with the location of the hypervariable region in other ras proteins. Northern (RNA) analysis was used to study expression of the three MRAS genes in relation to cell development. Gene-specific probes for two of these genes, MRAS1 and MRAS3, hybridized to different 1.3-kb mRNA transcripts. The accumulation of these transcripts depended on the developmental stage, and this pattern was different between the two MRAS genes. No transcript for MRAS2 was detected in the developmental stages examined. The unique patterns of MRAS transcript accumulation suggest that individual MRAS genes and proteins may play distinct roles in cell growth or development.The fungus Mucor racemosus is a saprophytic zygomycete that provides a useful system for studying eucaryotic cell differentiation. It is a relatively simple organism with a small genome (1.6 x 107 bp per haploid genome [34]) and an available transformation system (36) which facilitate molecular genetic investigations. M. racemosus is dimorphic: it exhibits filamentous (hyphal) growth in the presence of oxygen, but under anaerobic conditions with a fermentable hexose available, the organism grows as a single-celled, budding yeast (4). The fungus also undergoes morphogenesis during sporulation and germination of two asexual spore types, sporangiospores and arthroconidia, and a sexual spore called a zygospore.Morphogenesis in M. racemosus is correlated with changes in cyclic AMP (cAMP) and phospholipid metabolism (4,10,13,14,(21)(22)(23) large increase in phospholipid metabolism during yeast-tohypha morphogenesis, especially in the turnover of phosphatidylethanolamine and phosphatidylinositol (PI) (13). Cerulenin prevents phospholipid turnover and yeast-to-hypha morphogenesis under conditions in which cell growth continues at rates typical of yeast cells.The implication of cAMP and PI as secondary-messenger molecules in signal transduction pathways involving ras proteins, the ubiquity of ras proteins in eucaryotes, and the putative role of ras in cell differentiation and proliferation (2, 2...

Sporulation, spore germination, and yeast-hypha dimorphism in the filamentous fungus Mucor racemosus provide useful model systems to study cell development in eucaryotic cells. Three RAS genes (MRAS1, MRAS2, and MRAS3) from M. racemosus have been cloned, and their nucleotide sequences have been determined. The predicted amino acid sequences and the sizes of the three MRAS proteins exhibit a high degree of similarity with other ras proteins, including that encoded by H-ras, which have been implicated in regulation of proliferation and development in eucaryotic cells by mediating signal transduction pathways. The MRAS proteins show conservation of functional domains proposed for ras proteins, including guanine nucleotide interaction domains, an effector domain, a binding epitope for neutralizing antibody Y13-259, and the COOH-terminal CAAX box, which is a site of thiocylation and membrane attachment. Amino acid sequences unique to each MRAS protein occur adjacent to the CAAX box, consistent with the location of the hypervariable region in other ras proteins. Northern (RNA) analysis was used to study expression of the three MRAS genes in relation to cell development. Gene-specific probes for two of these genes, MRAS1 and MRAS3, hybridized to different 1.3-kb mRNA transcripts. The accumulation of these transcripts depended on the developmental stage, and this pattern was different between the two MRAS genes. No transcript for MRAS2 was detected in the developmental stages examined. The unique patterns of MRAS transcript accumulation suggest that individual MRAS genes and proteins may play distinct roles in cell growth or development.