Regulatory mechanisms in the expression of cell cycle dependent genes

There are several prominent features of cell cycle dependent gene regulation which are apparent from the data reviewed here. First, almost all of the genes studied are regulated by a combination of transcriptional and post transcriptional mechanisms. Thus, the regulation of mRNA levels through the cell cycle is a complex process, with control at many different levels. This is not surprising, if we keep in mind that the modulation of these mRNAs at the proper times may be critical to cell division. Secondly, there does not appear to be a common theme in the regulation of the genes discussed here. It appears as if each gene will be regulated by its own specific mechanism. Table 3 shows the serum responsive sequences which have been identified so far, and they are all different; there does not appear to be a consensus sequence yet for a serum response element. The identification of more such sequences should be forthcoming, and should give us a better idea of the general and specific nature of growth factor regulation of gene expression. We know less about the regulation of genes at the posttranscriptional level than about their transcription. We still have very little information about the specific sequences in mRNA which render it susceptible to degradation. Elucidation of such sequences, such as the AU rich region in GM-CSF mRNA should help us to understand serum and growth-factor gene regulation, since this means of control is as widespread, and probably as important as transcriptional control. We have only just begun to understand the mechanisms controlling the expression of CCD genes. The next few years should bring a great increase in our knowledge of these processes.