The H3.H4 tetramer forms the top half of the histone octamer. Histone H3.H4 tetramers occupy a key position in the nucleosome by binding the middle and both ends of the DNA

#Id: 2417

#Unit 3. Fundamental Processes

Termination requires that all hydrogen bonds holding the RNA–DNA hybrid together must be broken, after which the DNA duplex reforms.

#Id: 2418

#Unit 3. Fundamental Processes

he ability of the enzyme to terminate in vitro, however, is strongly influenced by parameters such as the ionic strength and temperature at which the reaction is performed.

#Id: 2419

#Unit 3. Fundamental Processes

Termination depends on the RNA product and is not determined simply by scrutiny of the DNA sequence during transcription.

#Id: 2420

#Unit 3. Fundamental Processes

Intrinsic terminators—that is, those that do not require auxiliary rho factor (r), as described shortly—require a GC–rich hairpin to form in the secondary structure of the RNA being transcribed.

#Id: 2421

#Unit 3. Fundamental Processes

Rho-dependent terminators are defined by the need for addition of rho factor in vitro, and mutations show that the factor is involved in termination in vivo.

#Id: 2422

#Unit 3. Fundamental Processes

Point mutations that reduce termination efficiency usually occur within the stem region of the hairpin, replacing GC base pairs with weaker AT base pairs, or in the U-rich sequence, supporting the importance of these sequences in the mechanism of termination.