Response to: The Schizosaccharomyces pombe imprint — nick or ribonucleotide(s)?

consistent with a one-ribonucleotide imprint; moreover, the authors do not attempt to detect molecules with a 5′ ′ end corresponding to a one-nucleotide gap. In addition, if the strand were broken due to hydrolysis of an RNA imprint, the 5′ ′ end would be devoid of any ribonucleotide(s); therefore, an RNase T2 control digestion (presented in the paper) is not expected to affect the outcome. Lastly, all the experiments aimed to exclude the presence of molecules with a gap at mat1 were performed using a mutant strain. In this strain a PstI restriction site was introduced at the site of the imprint, mutating one of the putative ribonucleotide positions. Thus, in this strain the imprint might always consist of one ribonucleotide, and as a consequence only a nick will be detected. In conclusion, Kaykov and Arcangioli's data are fully compatible with the proposed RNA nature of the imprint. The only unifying explanation for all the data available at this point is that cellular enzymatic activities gain access to the imprint during some DNA purification methods, converting an RNA imprint into the nick with the 3′ ′ and 5′ ′ hydroxyl groups detected by Kaykov and Arcangioli [4]. A programmed strand-specific and modified nick in S. pombe constitutes a novel type of chromosomal imprint. Vengrova and Dalgaard believe our observations are better explained by their ribonucleotide(s) replacement model [1] rather than by the dephosphorylated nick model we proposed [2]. Here we wish to review the two models in the context of previously published work and discuss the observations made in the accompanying letter. Recently, Vengrova and Dalgaard stated that the imprint can be purified intact, or cleaved in some conditions. We believe it may be purified as cleaved in all conditions, since we were never able to isolate intact imprinted mat1 upper strand. Furthermore, they propose that the imprint is composed of either one or two ribonucleotides, and that starting from an intact or nicked molecule, treatments with RNase T2 or NaOH remove one ribonucleotide from the 5′ ′-end mat1-distal upper strand, converting the imprint into a gapped molecule [1]. Our recent work [2] was designed to further analyze the molecular nature of the imprint and to directly challenge the nick and RNA models. A PstI site was inserted at mat1, and shows that a nick is located at a precise and fixed position, as observed in the wild-type strain, and …