We have previously reported a molecular and cytogenetic characterization of three different
5S rDNA clusters in the sea urchin Paracentrotus lividus and recently, demonstrated the presence of
high heterogeneity in functional 5S rRNA. In this paper, we show some important distinctive data on 5S
rRNA transcription for this organism. Using Single Strand Conformation Polymorphism (SSCP)
analysis, we demonstrate the existence of two classes of 5S rRNA, one which is embryo-specific and
encoded by the smallest (700bp) cluster and the other which is expressed at every stage and encoded
by longer clusters (900 and 950bp). We also demonstrate that the embryo-specific class of 5S rRNA is
expressed in oocytes and embryonic stages and is silenced in adult tissue and that this phenomenon
appears to be due exclusively to DNA methylation, as indicated by sensitivity to 5-azacytidine, unlike
Xenopus where this mechanism is necessary but not sufficient to maintain the silenced status
