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
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.
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.
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.