Sequence and Secondary Structure of the 5′ External Transcribed Spacer of Mouse pre-rRNA

We report the sequence of the 4006-nucleotide 5' external transcribed spacer (5'ETS) of the mouse ribosomal primary transcript. These data complete the sequence of the 13.4-kb mouse rRNA gene, thus providing a mammalian rRNA gene structure, in addition to yeast and Xenopus. The mouse 5'ETS displays a highly biased base content (very high in GC and particularly low in A), closely similar to the other transcribed spacers of the mouse ribosomal gene. This region seems to have accumulated sequence variation relatively rapidly during vertebrate evolution, with the possible insertion in rodents of sequences structurally similar to retroposons. About half the length of the mouse 5'ETS can fold into a giant and highly stable secondary structure, which is probably evolutionarily conserved in mammals and which could play an important role in the higher-order organization of mammalian pre-ribosomes.