Orc2-5 binds strongly; Orc6 binds weakly to ARS and has a nuclear localization signal that must be activated by the cyclin/ CDK kinase during the G1 to S transition.

TLS Online TPP Program

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#Unit 2. Cellular Organization

Effects of mutations in the three lac operators

All three operators are essential for effective repression.

TLS Online TPP Program

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#Unit 2. Cellular Organization

OC are constitutive because repressor protein is unable to bind; this allows RNA polymerase to have unrestrained access to the promoter. 

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#Unit 2. Cellular Organization

The Oc mutations are cis-acting/cis dominant, because they affect only the contiguous set of structural genes.

 

TLS Online TPP Program

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#Unit 2. Cellular Organization

The DNA-binding domain occupies residues 1–59. It contains two a-helices separated by a turn. This is a common DNA-binding motif known as the HTH (helix-turn-helix); the two a-helices fit into the major groove of DNA

TLS Online TPP Program

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#Unit 2. Cellular Organization

Promoter mutations are also cis-acting. If they prevent RNA polymerase from binding at P-lac, the structural genes are never transcribed.

TLS Online TPP Program

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#Unit 2. Cellular Organization

Mutants that cannot be expressed at all are called uninducible. Mutants that are continuously expressed are called constitutive mutants.