<P> The second control mechanism is a response to glucose, which uses the catabolite activator protein (CAP) homodimer to greatly increase production of β - galactosidase in the absence of glucose . Cyclic adenosine monophosphate (cAMP) is a signal molecule whose prevalence is inversely proportional to that of glucose . It binds to the CAP, which in turn allows the CAP to bind to the CAP binding site (a 16 bp DNA sequence upstream of the promoter on the left in the diagram below, about 60 bp upstream of the transcription start site), which assists the RNAP in binding to the DNA . In the absence of glucose, the cAMP concentration is high and binding of CAP - cAMP to the DNA significantly increases the production of β - galactosidase, enabling the cell to hydrolyse lactose and release galactose and glucose . </P> <P> More recently inducer exclusion was shown to block expression of the lac operon when glucose is present . Glucose is transported into the cell by the PEP - dependent phosphotransferase system . The phosphate group of phosphoenolpyruvate is transferred via a phosphorylation cascade consisting of the general PTS (phosphotransferase system) proteins HPr and EIA and the glucose - specific PTS proteins EIIA and EIIB, the cytoplasmic domain of the EII glucose transporter . Transport of glucose is accompanied by its phosphorylation by EIIB, draining the phosphate group from the other PTS proteins, including EIIA . The unphosphorylated form of EIIA binds to the lac permease and prevents it from bringing lactose into the cell . Therefore, if both glucose and lactose are present, the transport of glucose blocks the transport of the inducer of the lac operon . </P> <P> The lac repressor is a four - part protein, a tetramer, with identical subunits . Each subunit contains a helix - turn - helix (HTH) motif capable of binding to DNA . The operator site where repressor binds is a DNA sequence with inverted repeat symmetry . The two DNA half - sites of the operator together bind to two of the subunits of the repressor . Although the other two subunits of repressor are not doing anything in this model, this property was not understood for many years . </P> <P> Eventually it was discovered that two additional operators are involved in lac regulation . One (O) lies about - 90 bp upstream of O in the end of the lacI gene, and the other (O) is about + 410 bp downstream of O in the early part of lacZ . These two sites were not found in the early work because they have redundant functions and individual mutations do not affect repression very much . Single mutations to either O or O have only 2 to 3-fold effects . However, their importance is demonstrated by the fact that a double mutant defective in both O and O is dramatically de-repressed (by about 70-fold). </P>

Which of the following increases the rate of transcription initiation in the lac operon model