Identification of a repeated sequence in the genome of the sea urchin which is traescribed by RNA polymerase III and contains the features of a retroposon

摘要

A repeated sequence element which is located about 200 nuoleotides upstream from the protein-coding portion of the muscle actin gene (probably within a large 5′ intron) in the genome of the sea urchin, Strongylocentrotus purpuratus has been characterized, and shown to contain the sequence features which indicate that it has been transposed by means of an RNA intermediate. This retroposon-like sequence, SURF1-1, is a member of a family which is dispersed and repeated about 800 times in the genome, referred to as SURF1 (sea urchin retroposon family 1). In vitro transcription of this sequence by RNA polymerase III defines a 300 nucleotide transcription unit which is bounded by a short direct repeated sequence. The 3′ end of this unit contains a simple 21 nucleotide A+T?rich sequence characteristic of retroposons, and a consensus B box portion of an internal RNA polymerase III promotor is located 60 to 80 nucleotides downstream from the two sites of transcription initiation. This sequence also contains a 40 nuoleotide region that is related to several tRNA sequences (containing the B box), and a 79 nuoleotide sequence which is homologous to a repeated sequence previously shown to be present within the 3′ untranslated portions of the Spec1 and Spec2 mRMAs of this species (1). Analysis of transcripts of this sequence family in RNA from several embryonic stages indicates that its expression is highest at 11 hours postfertilization (about 128 cells) and drops as development procedes. Furthermore, most or all, transcription of this sequence family in nuclei isolated from 11 hour embryos is by RNA polymerase III, and is from the same strand which is transcribed in vitro.