RNA polymerase B from an α‐amanitin resistant mouse myeloma cell line

Mutant cell lines resistant to the cytotoxicity of cy-amanitin have been established from rat myoblasts [l] , Chinese hamster ovary [2,3] and BHK-T6 hamster cells [4]. All these mutant cell lines exhibit an altered form of RNA polymerase B under in vitro conditions which is more resistant to the action of cw-amanitin than the RNA polymerase activity isolated from wild type cells. The finding of Somers et al. [5] that the RNA polymerase activity in the a-amanitin resistant rat myoblast cell line L6 appears to be both of the sensitive and the resistant form, suggested that resistance to the toxin is a phenotypically dominant trait [ 1 ] . In the case of the a-amanitin resistant Chinese hamster ovary (CHO) cells however, the gene coding for the resistance to the toxin seems to be in a hemizygous condition since only resistant enzyme was found in these CHO cells [2]. In this paper we report the isolation and biochemical characterisation of an cr-amanitin resistant mutant selected from the mouse myeloma cell line MOPC 104 E. In contrast to the cu-amanitin resistant mutants described by other investigators which appear to be polyploid for the gene coding for the a-amanitin binding subunit we report here the finding of a mutant cell line which is diploid with regard to the synthesis of resistant and sensitive RNA polymerase B enzyme. 2. Materials and methods