Expression of a gene trap reporter construct in a subset of cells in embryonic sites of hematopoiesis: Evidence for alternative rRNA production in hematopoietic cells

Three mouse lines were generated from independent gene trap events in embryonic stem cells. These lines express a βgeo reporter gene in a subset of cells at sites of embryonic hematopoiesis. The 5' breakpoints of all three lines were found to lie in 45S ribosomal RNA transcription units. Expression was apparently linked to metabolic activity in these cells, since the kinetics of expression during embryogenesis matched that of cycling cells with colony forming unit spleen (CFU-S) potential. Expression was not seen in adult tissues unless the animals were treated with hydroxyurea, inducing synchronous entry of quiescent CFU-S into the cell cycle. Our results suggest that there is a subset of hematopoietic stem cells, which when actively proliferating, express the SAβgeo reporter construct from RNA polymerase I transcription units.