A LINKED 5S rRNA GENE IN SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES, PHAEOPHYCEAE)1

Abstract

ABSTRACT The complete nucleotide sequence of the transcribed and nontranscribed regions of the ribosomal RNA (rRNA) gene in the brown alga Scytosiphon lomentaria (Lyngbye) Link (Scytosiphonales, Phaeophyceae) is presented. Each rDNA repeating unit is about 8.3 kb in length and contains a 5S rRNA gene sequence in the same repeating unit as the 18S, 5.8S, and 25S rDNA. The 5S rDNA is located downstream from the 25S and arranged in the same transcriptional direction as the larger rDNAs. The 5S rDNA of Scytosiphon lomentaria shares 96.9–100% sequence similarity with 5S rRNA gene sequences of other brown algal species. Based on the very high sequence similarity of the Scytosiphon 5S rDNA to those of other chromophytes, the presence of a RNA polymerase III termination site (TTTTT) in the 3’flanking region of 5S, and indirect evidence of the lack of an unlinked 5S rDNA, we conclude that the linked 5S rRNA gene in Scytosiphon is functional. This is the first report of a presence of linked 5S rRNA gene in multicellular photosynthetic eukaryotes. Recent reports of the presence of linked 5S rRNA genes in all eukaryotes are reviewed, and the possibility that the Chromophyta lineage originally had a linked 5S rDNA as ancestor is discussed.