Short answer section. A helper element provides a function needed by a defective element in order to transpose or to grow. For example enhancer trap P-elements no longer carry transposase and require a helper to provide the enzyme. The CD4 and CCR-5 proteins together constitute the primary HIV receptor. State whether the following best apply to E. coli lac , or yeast GAL regulation, or whether they are equally applicable.
Inducer inactivates a repressor, thus allowing an activator to function.
Applies to both. In lac , inducer inactivates LacI and CAP-cAMP can stimulate transcription. In the yeast GAL system, inducer inactivates GAL80 uncovering the activation domain of GAL4 and transcription of the GAL genes occurs. Mutations in the activator can bypass the effect of uninducible repressor mutants. Applies to yeast GAL .