CHLOROPLAST DNA INHERITANCE Uniparental inheritance of organelle DNA is a ubiquitous feature of sexual eukaryotes, yet the underlying mechanisms that govern organelle DNA transmission are poorly understood. In Chlamydomonas reinhardtii the chloroplast DNA (cpDNA) of meiotic progeny is inherited exclusively from the parent that carries the plus mating type allele. Although both plus and minus parents contribute cpDNA to the zygote, the minus parental cpDNA is selectively destroyed within hours of mating.   While the prevailing model for uniparental cpDNA inheritance invokes a methylation-restriction system, our work on cpDNA methylation rules out that model and instead points to a role for methylation in regulating cpDNA replication in germinating zygotes. This finding adds a new dimension to the cpDNA inheritance problem and reopens the question of how selective destruction of mating type minus cpDNA is specified in zygotes. Our work on uniparental inheritance brings together several important problems of chloroplast molecular biology that have not been extensively addressed in higher plant systems. These include cpDNA replication, copy number control, and the roles played by proteins that package cpDNA into higher order structures or nucleoids.   PROJECTS Methyltransferase genes Genetic screens Nucleoids Replication of cpDNA CELL SIZE CONTROL Our recent discovery of an RB (retinoblastoma tumor suppressor) homologue in Chlamydomonas presents new opportunities to investigate the RB pathway in a model eukaryotic unicell with established genetic, molecular, cytological and biochemical techniques. The phenotype of a deletion mutant supports a model whereby the Chlamydomonas RB protein (Mat3p) acts as a size-dependent repressor of cell cycle progression. We can now use a variety of methods to test this model and determine how information on cell size is transmitted through the RB pathway to control the cell cycle. We can also begin to ask how the Mat3p pathway has been evolutionarily modified to generate the altered cell cycles that are integral to the developmental programming of closely related multicellular algae. PROJECTS Phosphorylation of Mat3p (Rb) in response to cell size Transcriptional control by Mat3p Analysis of pathway components Genetic screens Evolution of the size control pathway in multicellular relatives