Structure and expression of a second sea urchin U1 RNA gene repeat.

There are two tandemly repeated sets of U1 RNA genes in the sea urchin L. variegatus. Each of these genes is present in a 1.4 kb repeat defined by a HindIII site about 450 bases 5' to the gene. The sequences of a member of both repeating units (U1.1 and U1.2) has been determined. The repeats are nearly identical for 550 nucleotides 5' to the gene but show great divergence starting 30 nucleotides 3' to the gene, just after the CAAAGAAAGAAAA sequence thought to be required for 3' end formation. The other boundary between the conserved and non-conserved sequences is a polypyrimidine sequence (on the strand which codes for U1 RNA). Both of these repeats are expressed in blastula stage embryos, as judged by transcription of unique sequences 3' to the gene in nuclei isolated from blastula stage embryos. At least some of the two types of repeats are interspersed, since representatives of both repeat types on a single gamma phage isolated from a gene library. The sequence of the U1 RNA in L. variegatus eggs and embryos corresponds to the sequence of the U1 repeat.