Effects of ionizing radiation on RNA metabolism in cultured mammalian cells II. Patterns of precursor incorporation into messenger-related RNAs of exponentially growing and synchronized populations

Abstract

During the later portion of the division delay period which follows exposure of cultured Chinese hamster cells to 800 rads ionizing radiation, there is an increased incorporation of [ 3H]uridine (relative to control cells) into heterogeneous nuclear RNA (HnRNA) and polysomal mRNA without a corresponding increase of incorporation into informosomal messenger-like RNA. The nature of this effect has been further investigated. 1. 1. The increased net incorporation reflects an increase in incorporation rate, not a decrease in breakdown; 5-, 10- and 20-min pulses result in the same relative incorporation into HnRNA of irradiated and control cells. The rate of mRNA decay in actinomycin-treated irradiated and control populations is comparable. 2. 2. The magnitude of the effect is the same after 800 and 1600 rads. 3. 3. The effect is not apparent when synchronized cells are irradiated in G 1 and the incorporation rates in late interphase of control and irradiated populations are measured. It is concluded that the effect of irradiation on relative rates of uridine incorporation into messenger-related RNAs is a reflection of (1) the fact that the irradiated population is arrested in G 2 (period between end of S (the DNA synthetic period) and beginning of mitosis) while most of an exponential control population is in G 1 (period between mitosis and DNA synthesis) and (2) the incorporation pattern differs in G 2 and G 1, the incorporation into HnRNA and polysomal mRNA being greater relative to that into informosomal messenger-like RNA in G 2 than in G 1.