Introduction to Genetic Analysis 595

Introduction to Genetic Analysis 595 - 44200_18_p575-610...

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its expression limited to a restricted region of the A–P axis, each gap gene organizes the formation of a spa- tially localized set of segments. How is expression confined to just the proper location? BCD or HB-M protein or both bind to enhancer elements of the pro- moters of the gap genes, thereby regulating their tran- scription. For example, transcription of one gene, Krüp- pel (Kr), is repressed by high levels of the BCD transcription factor but is activated by low levels of BCD and HB-M. In contrast, the knirps (kni) gene is re- pressed by the presence of any BCD protein but re- quires low levels of the HB-M transcription factor for its expression. Thus the gap-gene regulatory sequences have been exquisitely “tuned” by evolution each to have a unique sensitivity to the concentrations of the transcription factors of the A–P positional-information system. The result of this regulation is that the kni gene is expressed more posteriorly than is Kr (Figure 18-23a). In general, through differences in their regulatory element
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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