Molecular characterisation of plant U14 small nucleolar RNA genes: closely linked genes are transcribed as polycistronic U14 transcripts.

U14snoRNAs are highly conserved eukaryotic nucleolar small RNAs involved in precursor ribosomal RNA processing. In vertebrates, U14snoRNAs and a number of other snoRNAs are transcribed within introns of protein coding genes and are released by processing. We have isolated potato and maize genomic U14 clones using PCR-amplified plant U14 probes. Plant U14s show extensive homology to those from yeast and animals but contain plant-specific sequences. One of the isolated maize clones contains a cluster of four U14 genes in a region of only 761 bp, confirming the close linkage of U14 genes in maize, potato and barley as established by PCR. The absence of known plant promoter elements, the proximity of the genes and the detection of transcripts containing linked U14s by RT-PCR indicates that some plant U14snoRNAs are transcribed as precursor RNAs which are then processed to release individual U14s. Whether plant U14snoRNAs are intron-encoded or transcribed from novel promoter sequences, remains to be established.