C-mos expression

Although testicular and ovarian germ cells clearly represent the principal sites of c-mos transcription, lower levels of c-mos expression have been reported in some somatic tissues and cell lines. Propst et al. (1985, 1987) reported low levels of c-mos transcripts in mouse embryos and... 

Figure 6. The c-mos negative regulatory element (NRE). Nucleotide positions of the NRE are shown relative to the spermatocyte transcription start site, taken as 280 base pairs upstream of the c-mos ATG (see Fig. 4). The endpoints of the NRE are defined by deletions that allow c-mos expression in NIH 3T3 and other somatic cells. Mutations of the sequences designated by boxes 1,2, and 3 also allow c-mos transcription in NIH 3T3 cells, indicating that these sequences represent functional elements within the NRE. Boxes 1 and 2 are similar to sequences upstream of the protamine (Prot) promoter that inhibit in vitro transcription in HeLa cell extracts. A sequence just upstream of box 2 is also similar to a putative repressor-binding site in the regulatory region of Pgk2.

It should be noted that the NRE defined in these experiments is distinct from the previously described c-mos UMS sequence (Blair et al., 1984 Wood et al., 1984). The UMS is located approximately 1.4 kb upstream of the c-mos spermatocyte promoter and was identified because it blocked activation of c-mos transforming potential by insertion of retroviral promoters. It is thought to act as a transcriptional terminator, blocking transcription of c-mos initiated at upstream sequences. However, both the spermatocyte and oocyte transcription initiation sites are substantially downstream of the UMS. Moreover, the presence or absence of the UMS does not affect c-mos expression in either microin-jected oocytes (Pal et al., 1991) or transfected NIH 3T3 cells (Zinkel et al., 1992). It thus appears unlikely that the UMS functions as a negative regulator of c-mos transcription from either the spermatocyte or oocyte promoters in somatic cells. 

Also in need of further study is the function of Mos in male germ cells. It appears that Mos is expressed prior to meiosis of spermatocytes, consistent with the possibility that it acts in the initiation of meiosis and/or during progression from meiosis I to meiosis II. However, c-mos expression continues in postmeiotic spermatids, where it does not induce metaphase II arrest. This may be due to the absence of other components of cytostatic factor, but the role of c-mos expression in postmeiotic male germ cells remains unclear. 

Pal, S. K., Zinkel, S. S., Kiessling, A. A., and Cooper, G. M. (1991). c-mos expression in mouse oocytes is controlled by initiator-related sequences immediately downstream of the transcription initiation site. Mol. Cell. Biol. 11 5190-5196. 

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