Fig. 5. L1 EN is inhibited by RNA. (A) L1 EN nicking activity was assayed by following conversion of the quickly migrating supercoiled KS–plasmid to the slowly migrating open-circular form. (B) L1 EN nicking of a supercoiled plasmid was challenged with 10-fold dilutions (100 µM–100 nM; 100 µM–10 nM for G₂₀) of the indicated RNA oligo. Unlike the related CCR4 nuclease (Chen et al., 2002), L1 EN has neither polyA-specific RNA exonucleolytic activity, nor any detectable nucleolytic activity on RNA (data not shown). (C) Quartet structures (as assayed by differential light absorbance and thermal melting) are disrupted when polyI is converted to a lithium salt. (D) Quartet structures are not required for inhibition of L1 EN nicking. Ten-fold dilutions (100 ng/µl–10 pg/ml); no difference was seen even at two-fold dilutions (data not shown). sc., supercoiled; oc., open circular.