inhibition constant (K ) which was determined for calf 



-9 

 thymus RNA polymerase II to be 3-10 x 10 M (Cochet- 



Meilhac and Chambon, 1974). The amanitin mediated inhi- 

 bition of RNA polymerase II results in cell death due to 

 the lack of messenger RNA (mRNA) synthesis. RNA polymer- 

 ases II are those polymerases identified as being respon- 

 sible for most of the mRNA synthesis in eucaryotic cells 

 (Chambon, 1975). Evidence for this is derived from several 

 observations. The synthesis of poly-adenosine containing 

 heterogenous nucleoplasmic RNA (hnRNA) by HeLa cells is 

 inhibited by low concentrations of a-amanitin (Zylber and 

 Penman, 1971). HnRNA is a precursor in eucaryotic cells 

 to those species of RNA that have been identified as func- 

 tional mRNA (Brawerman, 1974). Low concentrations of a- 

 amanitin have also been shown to inhibit the synthesis of 

 adenovirus mRNA in human KB cells (Ledinko, 1971) but do 

 not affect the synthesis of mRNA by viruses which use virus- 

 coded RNA-dependent RNA polymerases that are amanitin 

 resistant (Wieland and Faulstich, 1978). 



Mammalian cell lines resistant to the effects of a- 

 amanitin have provided additional verification of the 

 molecular target of a-amanitin. Isolation and purification 

 of RNA polymerase II from a-amanitin resistant Chinese 

 hamster ovary (CHO) cells yielded an enzyme resistant to 

 u-amanitin (Chan et al., 1972). Amanitin resistant mutants 

 of BIIK-T6 hamster cells (Amanti et al., 1975), rat myoblasts 

 (Somers et al . , 1975), mouse myeloma MOPC 104 E cells 



