The boundaries of partially edited transcripts are not conserved in kinetoplastids: implications for the guide RNA model of editing.

We have studied partially edited molecules for the cytochrome-c oxidase subunit III (COIII) transcript from two species of the insect trypanosome Herpetomonas. We found unexpected patterns of editing, in which editing does not proceed strictly 3' to 5', in 24 of 61 partially edited clones. A comparison of the partially edited molecules between the two kinetoplastid species revealed an 8- to 10-nt shift in precisely defined editing boundaries, sites at which editing pauses before binding of the next guide RNA after formation of a stable duplex between a guide RNA and mRNA. This suggests that the region of base pairing between individual guide RNAs and the COIII transcript is not strictly conserved in kinetoplastids, implying gradual evolution of the editing process.