Inhibitors to mRNA synthesis (Transcription) are

Inhibitors to mRNA synthesis (Transcription) are Actinomycin-D, Rifampicin, also 5-Bromouracil.
Transcription inhibition is accompanied by notable changes in the biochemical properties of nuclear proteins such as histones and hnRNPs.
Actinomycin D is a transcription inhibitor that intercalates into DNA. Actinomycin D forms a very stable complex with DNA, preventing the unwinding of the DNA double-helix, thus inhibiting the DNA-dependent RNA polymerase activity.
Rifamycin inhibits bacterial DNA transcription into mRNA by inhibiting DNA-dependent RNA polymerase by binding its beta-subunit.
5-Bromouracil is used to treat neoplasms in the form of its nucleoside, 5-Bromo-2-deoxy-uridine. 6-Mercaptopurine is an analog of hypoxanthine and inhibits purine nucleotide synthesis and metabolism.