The structures that directly bind to the promoter are letter d (a structure) and letter c (a large structure that is about to attach to b).
In gene regulation, the identified structures play distinct roles in the process. Letter d, representing a structure, is crucial as it specifically binds to a particular part of the gene, likely involved in initiating or controlling gene expression. On the other hand, letter c, denoting a large structure that is about to attach to b, indicates an element that interacts with part b of the gene.
This interaction is significant as it implies a regulatory process where the large structure likely influences or modulates the activity of the gene by binding to its promoter region. Together, structures c and d directly contribute to the regulatory mechanisms governing gene expression by interacting with specific elements in the gene's upstream and promoter regions.