Molecular characterization at the RNA and gene levels of U3 snoRNA from a unicellular green alga, Chlamydomonas reinhardtii

A U3 snoRNA gene isolated from a Chlamydomonas reinhardtii (Cre) genomic li brary contains putative pol III-specific transcription signals similar to t hose of RNA polymerase III-specific small nuclear (sn)RNA genes of higher p lants. The 222 nt long Cre U3 snoRNA was immunoprecipitated by anti-gamma-m pppN antisera, but not by anti-m(2,2,7)G antibodies, supporting the notion that it is a RNA polymerase III transcript. Tagged Cre U3 snoRNA gene const ructs were expressed in Cre cells. Results of chemical and enzymatic struct ure probing of Cre U3 snoRNA in solution and of DMS modification of Cre U3 snoRNA under in vivo conditions revealed that the two-hairpin structure of the 5'-domain that is found in solution is no longer detected under in vivo conditions. The observed differences can be explained by the formation of several base pair interactions with the 18S and 5'-ETS parts of the pre-rRN A, A model that involves five intermolecular helices is proposed.