James Umen

Prior to his coming to the Danforth Center as a member of the Enterprise Rent-A-Car Institute for Renewable Fuels, Dr. Umen served as Assistant Professor of the Plant Molecular and Cellular Biology Laboratory at the Salk Institute for Biological Studies. He was a Postdoctoral Fellow in the Department of Biology at Washington University in St. Louis, Missouri and an instructor at Guilin Geology College in China. Dr. Umen received his Ph.D. in Biochemistry and Biophysics from the University of California – San Francisco and his B.S. in Biology from Stanford University, Palo Alto, California.

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Research

Areas of expertise in the Umen lab include cell size homeostasis and the RB tumor suppressor pathway, cell growth regulation in photosynthetic eukrayotes, and evolution of development complexity. Algae is used as a primary experimental system.

Cell Size Homeostasis and the RB Tumor Suppressor Pathway

Size homeostasis is a fundamental property of proliferating cells and is thought to be governed by cell size checkpoints. The multiple fission cell cycle of Chlamydomonas uncouples cell growth and division and allows us unique access to a size checkpoint mechanism. A key regulator of this checkpoint is the Chlamydomonas retinoblastoma (RB) tumor suppressor pathway, whose function in cell size and cell cycle regulation is a major focus of investigation.

Cell Growth Regulation in Photosynthetic Eukaryotes

Cell growth in eukaryotes requires the coordinate regulation of cytoplasmic biosynthetic processes with those in chloroplasts and mitochondria, semi-autonomous organelles that contain their own protein biosynthetic machinery. Chloroplasts from higher plants and green algae represent a large fraction of cellular biomass yet it is unknown how their growth is regulated with respect to cytoplasmic growth. The TOR (target of rapamycin) kinase signaling pathway is conserved in all eukaryotes where it functions as a nutrient-sensitive modulator of growth rates. We are using Chlamydomonas as a simple model for how TOR signaling contributes to coordinated growth control in photosynthetic eukaryotes.

Evolution of Developmental Complexity

Chlamydomonas reinhardtii belongs to a diverse clade of green algae, some of which have undergone a remarkable transition to multicellularity. The best-characterized of the multicellular relatives is Volvox carteri, a species that embodies many of the hallmarks of multicellular metazoans or plants. These include terminally differentiated somatic cells, reproductive stem cells, complex embryonic patterning, and formation of sexually dimorphic germ cells (eggs and sperm), none of which are present in its unicellular relative Chlamydomonas. Indirect evidence suggests that the RB tumor suppressor pathway might be coupled to germ/soma differentiation and to dimorphic germ cell production. Our current work is aimed at cloning and characterizing the mating locus that is the genetic determinant for sperm and egg formation.

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