37rc Cells Rapidly Recover Their Rna Synthesis after Inhibition with High Doses of Actinomycin D

Actinomycin D (AD) produces in most cultured cell lines a complete and essentially irreversible inhibition of DNA-dependent RNA synthesis (Reich et al ., 1961 ; Girard et al ., 1964) . Several RNA species are reported to be AD resistant (Paul and Struthers, 1963 ; Roberts and Newman, 1966 ; Penman et al ., 1968 ; Perry et al ., 1970; Dubin and Montenecourt, 1970) . On the cellular level, a number of factors may be responsible for the different AD sensitivity of various cell systems (Djaczenko et al ., 1970; Schluederberg et al ., 1971) . In the present study we demonstrate that in 37RC cells rapid recovery of RNA synthesis takes place after inhibition with high doses of AD, possibly because they retain a relatively small intracellular pool of the antibiotic. We also compare the AD sensitivity of 37RC cells with that of HeLa cells in several experimental conditions . We used four different strains of the 37RC line (Cercopithecus kidney cells) and the S3 strain of the HeLa line. All lines were kept in monolayers in one of the following culture media : (a) Hanks' balanced salt solution (Hanks and Wallace, 1949) ; (b) minimal essential medium (Eagle, 1959) ; or (c) Synthetic 199 (Difco Laboratories, Inc., Detroit, Mich .). Each medium was supplemented with 5% hydrolized lactalbumin, 5% or 10% inactivated calf serum, and 100 OU/ml of penicillin and 100μg/ml of streptomycin. We used also mycoplasma-free lines of 37RC and HeLa S3 maintained in Synthetic 199 medium with addition of 100 jug/ml of kanamycin . This antibiotic was removed from the culture medium 2 hr before the beginning of the experiments . We performed