The antibiotic rifamycin blocks synthesis of rna from a dna template. the most immediate effect would be to?

The most immediate effect would be inhibition of protein synthesis.

Explanation:

The process of protein synthesis, also called translation, is based on the reading of messenger RNA and on the union of amino acids corresponding to the codon sequence (sequence of three nitrogenous bases) present in this RNA. In protein synthesis, therefore, the conversion of information contained in RNA molecules into proteins occurs.

If the antibiotic rifamycin inhibits RNA synthesis in a bacteria it means that protein synthesis of that bacteria will also be inhibited. The bacteria will not have the proteins responsible for the functioning of your body and will eventually die.