Rifamycin and actinomycin D are two antibiotics derived from the bacterium Streptomyces. Rifamycin binds to the b subunit of E. coli RNA polymerase and interferes with the formation of the first phosphodiester bond in the RNA chain. Actinomycin D bind to double stranded DNA by intercalation (slipping in between neighboring base pairs).

a) Which of the four stages in transcription would you expect rifamycin to affect primarily? Explain why.

b) Which of the four stages in transcription would you expect actinomycin D to affect primarily? Explain why.

c) Which of the two inhibitors is more likely to affect RNA synthesis in cultured human liver cells? Explain why.

d) Which of the two inhibitors would be more useful for an experiment in which it is necessary to block the initiation of new RNA chains without interfering with the elongation of chains that are already being synthesized? Explain why