Somatic cell expression of the mos proto-oncogene is cell cycle regulated: Highest RNA expression in the G2 phase

c-mos expression, which occurs at relatively high levels in male and female germ cells, plays an important role in oocyte meiotic maturation. The c-mos proto-oncogene product (c-Mos) is necessary and sufficient to initiate meiosis. It is also an essential component of the cytostatic factor (GSF), which is responsible for arresting vertebrate oocytes at the second meiotic metaphase possibly via stabilization of the maturation promoting factor (MPP). However, much less is understood about c-mos expression and function in somatic cells. We report here that c-mos transcripts can be detected in NIH 3T3 cells by the highly sensitive RNA-PGR method and by RNase protection assays. We found that expression of c-mos RNA is tightly controlled in a cell cycle-dependent manner with highest levels of transcripts (approximately 5 copies per cell) present in the G2 phase. The level of c-mos RNA in synchronized G0/G1 cells was undetectable, and that in S phase cells was extremely low. Similarly, only very low levels of c-mos RNA were detected in nocodazole-arrested M phase cells. The presence of contaminating G2 cells in the synchronized S phase and M phase populations as well as unsynchronized populations could account for the very low levels of c-mos transcripts detected and supports the interpretation that c-mos RNA is absent in all phases except G2. These results establish that c-mos expression is not restricted to germ cells, but instead indicate that c-mos RNA expression occurs during the G2 stage of the cell cycle in somatic cells. As in meiosis, c-mos may have a similar function in regulating cell cycle events in somatic cells particularly in controlling entry into mitosis via activation of MPF.