where pol l-leave is the concentration of the polymerases leaving the genomic promoters, O ,n and O ,nþ1 are the concentrations of a component (O) at time n and time nþ1 (in our numerical integration, time nþ1 À time n ¼ Dt), respectively. pol trc;nþ1 ¼ pol trc;n þ pol new trc;n À pol trcÀleave;n ð24Þ