[5S genes of the loach: determination of the primary structure of the transcription termination region and nontranscribed spacer].

The primary structure of a 5S gene spacer has been determined by sequencing three cloned 5S rDNA fragments of the loach genome. The region of the spacer adjacent to the 3'-termini of the gene structural part was shown to comprise an AT-rich sequence (24 bp long) including an oligo (T)6-9 block corresponding to the terminator of RNA polymerase III. The results supported our previous data about the 5S rRNA precursor synthesis during the transcription of cloned 5S rDNAs injected into oocyte nuclei. The comparative sequence analysis revealed a homology between the spacer region from -54 to -26 bp and the 5'-termini (1-18 bp) and the 3'-termini (108-117 bp) stretches of the gene coding region. In addition, both the coding part and nontranscribed spacer of 5S DNA include highly diversed repeats which are homologous to the 3'-termini sequence (111-+8 bp) of the gene. Apparently the nontranscribed spacer of loach 5S genes derived in evolution from numerous replicated 5S genes as a result of subsequent base elimination and substitution in the most of the 5S rRNA coding sequences.