A-P cardinal genes

A-P cardinal genes are also known as gap genes.

BCD or HB-M protein or both bind to enhancer elements of the promoters of the gap genes.

This binding ensures the regulation of their transcription.

For instance, Kruppel (kr) gene transcription is repressed by high levels of BCD ands activated by low levels of BCD and HB-M.

Knirps (kni) gene is repressed by the presence of BCD, yet it doesn’t require low levels of HB-M for expression.

The enhancer and promoter properties make sure that the kni gene is expressed more posteriorly than Kr

Gap genes can be expressed in a series of distinct domains which then become developmental fields. This is because we have promoters that are differentially sensitive to the concentrations of transcription factors in the A-P positional information system. Cells in distinct domains are thus committed to different fates.