Expression of B-Myb during mouse embryogenesis.

B-myb is a member of the myb family of nuclear sequence-specific DNA-binding proteins which has been highly conserved among vertebrates. B-myb has been implicated in the control of cell proliferation, particularly at the G1/S transition of the cell cycle. So far, most of the work on B-myb has been performed in immortalized cell lines. Since these cells might show aberrant behavior of genes involved in proliferation control we have begun to investigate the role of B-myb in normal cells. As a first step, we have studied the expression of B-myb during mouse development. Here, we show the B-myb is expressed at similar levels during all stages of embryogenesis. In situ hybridization reveals a tight linkage between B-myb expression and proliferative activity (as assessed by the expression of the S-phase specific histone H4 gene) in most tissues and throughout embryonic development. However, B-myb and histone H4 expression are uncoupled during spermatogenesis in the adult mouse. Histone H4 is expressed at high levels in the early spermatogenic progenitor cells but not in successive stages of sperm cell development. By contrast, the highest levels of B-myb expression are found during the intermediate stages of spermatogenesis. Furthermore, we have found that B-myb mRNA isolated from the testis differs in size from that of other tissues. The data presented here strongly support the notion that B-myb plays a general role during proliferation of most cells. Furthermore, our results raise the possibility that the function of B-myb in cells undergoing meiosis may be different from its role in cells dividing mitotically.