The preinitiation complex (PIC) assembles in a stepwise fashion on the promoter of genes to initiate transcription. The TFIID binds to the TATA box in order to begin the assembly of the TFIIA, recruiting other transcription factors and components needed in the PIC. Data suggests that pRb is able to repress transcription by both pRb being recruited to the promoter as well as having a target present in TFIID.
The presence of pRb may change the conformation of the TFIIA/IID complex into a less active version with a decreased binding affinity. pRb can also directly interfere with their association as proteins, preventing TFIIA/IID from forming an active complex.