Antigen pieces are held within the binding groove of a cell surface protein called the major histocompatibility complex (MHC) molecule which recognized by T cell

#Id: 2440

#Unit 3. Fundamental Processes

In some cases, a nonsense mutation in one gene of a transcription unit was found to prevent the expression of subsequent genes in the unit even though both genes had their own ribosome binding sites, an effect called polarity.

#Id: 2441

#Unit 3. Fundamental Processes

Rho-dependent termination sites within a transcription unit are usually masked by translating ribosomes, and therefore rho cannot act on downstream RNA polymerases. 

#Id: 2442

#Unit 3. Fundamental Processes

Nonsense mutations release ribosomes within the RNA of a multigene operon, though, enabling rho to terminate transcription prematurely and prevent expression of distal genes in the transcription unit even though their open reading frames contained wild-type sequences

#Id: 2443

#Unit 3. Fundamental Processes

The need for rho factor in vitro is variable. Some (rho-dependent) terminators require relatively high concentrations of rho, whereas others function just as well at lower levels. 

#Id: 2444

#Unit 3. Fundamental Processes

Different terminators require different levels of rho factor for termination and therefore respond differently to the residual levels of rho factor in the mutants (rho mutants are usually leaky).

#Id: 2445

#Unit 3. Fundamental Processes

The b subunit of RNA polymerase is implicated by two types of mutation. 

First, mutations in the rpoB gene can reduce termination at a rho-dependent site. 

Second, mutations in rpoB can restore the ability to terminate transcription at rho-dependent sites in rho mutant bacteria.