A full understanding of the function of an RNA molecule requires a detailed knowledge of its spatial folding. In recent years the pseudoknot fold has been added to the possible structures an RNA may adopt. Starting from a fully denatured RNA chain, the first structural element formed upon base pairing is the well-known hairpin or stem-loop structure (Figure 1A). Addition of a second stem region,

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A conserved secondary structure model has been proposed recently for the telomerase RNAs from six different Tetrahymena and one Glaucoma species (1). In this model four helical regions are present which are all supported by phylogenetic evidence. The templating domain is unpaired in these structures, in agreement with the observation that this region is available for basepairing with complementary DNA oligonucleotides (2). We note that the nucleotides of the loop of helix III are capable of basepairing with a stretch of four bases just upstream of this stem. The pseudoknot thus formed is present in all seven telomerase sequences, and is supported by covariations in three of the four basepairs involved (Table 1). Figure 1 gives the structure of the H-type pseudoknot we propose for the T. thermophilus (3).