Rates of transcript decay and synthesis in cultured cells. A) Decay rates of the c-fos-2 (open symbols) and c-fos (solid symbols) transcripts in NIH 3T3 (circles) and Hepa1-6 (triangles) cells. Data are the percentages of transcript molecules remaining after the addition of AmD. The half-life (t1/2) values calculated from the resulting decay lines are given for comparisons. B) Rates of transcript decay and synthesis in NIH 3T3 cells. Transcript synthesis rates (in molecules/min) were calculated from the basal amounts of transcript molecules (per pg of total RNA) and their estimated half-lives (in minutes). These calculations are based on the assumption that though mRNA degradation is a complex process, it follows first-order kinetics. The steady-state levels and rates of decay and synthesis of transcripts coding for both types of FosB protein (fosB and ΔfosB) and for heme oxygenase 1 (ho1), thioredoxin 1 (trx1) and superoxide dismutase 3 (sod3) are included for comparison.