Enhancers and Expression of Immunoglobulin Genes
Up to now in this chapter, we have ignored the problem of regulation of expression of the immunoglobulin genes. Consider the 300 or so kappa-type V regions. The cell would waste considerable resources on
Figure 20.11 An enhancer in the region between the J and C regions stimulatesa promoter lying in front of a V region segment after the V region has been joined to the C region.
the synthesis of messenger, and perhaps its translation, if each of these gene segments were transcribed. How then can the fusion of a V segment to a C segment activate transcription of the spliced gene and only the spliced gene? The explanation, of course, is that each V region contains a promoter, but lacks an enhancer for activation. The necessary en-hancers are located near the C genes (Fig. 20.11). Therefore, not only does gene splicing generate a substantial portion of the diversity in antibodies, but it also activates transcription of a spliced gene. Only after a V region has been placed near the C region is the enhancer sufficiently close to the promoter to activate it.
Like many enhancers, those serving the immunoglobulin genes are highly complex. Many different proteins bind to the enhancer. Some of these function in other tissues and must serve in a general way to activate. Others must be specific for cells of the immune system. Overall, however, the combination of enhancer proteins present in the cells of the immune system distinguishes these cells, and genes of the immune system are turned on only in the appropriate tissues.
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