Which is NOT a valid host for two-hybrid screening?
Classical two-hybrid relies on reconstitution of transcription factor function inside a living cell. Saccharomyces cerevisiae is the traditional host providing nuclear environment, reporter genes, and genetics. Bacterial two-hybrid adapted for E. coli uses repressor or activator reconstitution. Bacteriophage is not a cellular host but serves as display particle in phage display, not two-hybrid complementation. Although mammalian two-hybrid variants exist, standard screening for libraries is not routinely performed in human cells due to transfection efficiency and reporter complexity, making human cells non-standard for conventional yeast-based screening pipelines.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.