2762 Nucleic Acids Research, 2005, Vol. 33, No. 8 extent, pre-5S transcripts. Therefore, we cannot exclude that RNR1 could be partially responsible for processing these tails. This facet of RNR1 activity might require accessory pro- tein(s). By analogy to the E.coli degradosome, where the asso- ciation of PNPase with the RNA helicase RhlB allows it to turn over highly structured substrates (50,51), it is possible that RNR1 processivity is enhanced by its association with an RNA helicase. One candidate is the chloroplast RNA helicase encoded by VDL (52), which is essential for the function of all plastid types, as would be expected of an rRNA maturation factor. Whether the vdl mutant accumulates aberrant plastid transcripts remains to be reported. Arabidopsis contains three RNR genes Residual rRNA processing in rnr1 prompted us to search for additional members of the RNR superfamily encoded in the Arabidopsis nuclear genome, and two such genes were found. We have tentatively designated these RNR2 (At1g77680) and RNR3 (At2g17510). While experiments with the RNR N- terminal 100 amino acids fused to GFP conﬁrmed chloroplast targeting for RNR1, the putative RNR2 transit peptide (N- terminal 231 amino acid) targeted GFP to the nucleus and RNR3 targeting, using the N-terminal 194 amino acid of RNR3, resembled that of the untagged GFP control, suggest- ing that RNR2 and RNR3 might be components of the exo- some (Supplementary Figure 1) (23). Therefore, chloroplasts appear to contain only a single RNR family enzyme, as well as PNPase. Whether these enzymes catalyze redundant activities remains to be seen, but a pnp1/rnr1 double mutant is embryo lethal (S. Yehudai-Resheff, T. Bollenbach, R. Gutierrez and D. Stern, manuscript in preparation). SUPPLEMENTARY MATERIAL Supplementary Material is available at NAR Online. 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