This is a revised grant application to study global gene expression in Cryptosporidium parvum and in host cells infected with this parasite. Cryptosporidium parvum is an opportunistic pathogen in people with AIDS and is a common cause of diarrhea in children worldwide. The completed sequence of the C. parvum genome provides new opportunities for identifying drug targets and for research on this parasite on a genomic and functional level. In this project DNA microarrays will be used to study parasite gene regulation and the response of the host cell to this infection. Analysis of global parasite transcription, primarily during excystation, host cell invasion, and the initial phase of intracellular development will identify parasite biochemical pathways, which are differentially expressed during this phase of the life cycle. Analysis of the transcriptional response of infected host cells in culture and in animal models will identify mechanisms by which the host responds to the infection, and shed light on the molecular pathways leading to the intestinal symptoms of cryptosporidosis. This project is a collaborative effort between two research groups with complementary expertise in the biology and genomics of C. parvum and extensive collaborative experience. In Specific Aim 1 a comprehensive DNA microarray containing all of the genes of C. parvum will be developed. This task will take advantage of the genomic sequence by converting genomic information into a tool for identifying genetic processes associated with parasite development. In Specific Aim 2 microarrays developed under aim 1 will be used to probe global gene expression during oocyst excystation, host cell invasion, and first generation merogony. Specific Aim 3 will use commercially available human and mouse gene arrays to study the response of the host cell to C. parvum. A comparison of the host cell response to C. parvum type 1 and type 2 will be performed to probe the molecular basis of the observed difference in virulence between types.