Evidence that the (6-4) photolyase mechanism can proceed through an oxetane intermediate.

Using the analogue of TpT methylated at the 3′-end N3 position (Tpm3T), we demonstrate that when the oxetane/(6−4) pathway is precluded, water addition occurs at the 3′-end C6 position of the oxetane intermediate to provide its opening. Photoreversal of this (6−4) photoproduct C6 hydrate brings the first experimental evidence that the (6−4) photolyase repair can proceed through an oxetane intermediate.