Cap is O 157 chromogenic medium. The complex formed by the combination of cyclic adenosine monophosphate (cAMP) and receptor protein CRP(cAMPreceptorprotein) is CAP(cAMPactivatedprotein CAP). When E.coli grows in the medium lacking glucose, the amount of CAP synthesis increases, and CAP has the function of activating promoters such as lactose (Lac). Some CRP-dependent promoters lack the typical -35 region sequence characteristics (TTGACA) of general promoters. So it is difficult for RNA polymerase to bind to it. The existence of CAP can significantly improve the binding constant of enzyme and promoter. It is mainly manifested in the following two aspects: CAP can change the conformation of the promoter, interact with the enzyme, help the enzyme molecule to orient correctly, and then combine with the-10 region to replace the function of -35 region. CAP can also inhibit the binding of RNA polymerase to other sites in DNA, thus increasing the probability of binding to its specific promoter.