Two highly conserved transcribed regions in the 5S DNA repeats of the nematodes Caenorhabditis elegans and Caenorhabditis briggsae.

The 5S RNA genes of Caenorhabditis briggsae consist of approximately 65 copies of a 1 kb repeat unit and 20 copies of a related 0.7 kb repeat unit, organized in separate tandem clusters. DNA sequence comparisons with the 1kb 5S DNA repeat from the closely related nematode C. elegans show that the 5S RNA coding region is perfectly conserved. Both C. briggsae 1 kb and 0.7 kb repeats are also efficiently transcribed in vitro,suggesting that both represent functional 5S RNA genes. Surprisingly, a second block of 118 bp is also perfectly conserved between the 1 kb repeats,and is less well conserved in the 0.7 kb repeat. In C. elegans, this DNA is transcribed to produce and abundant 100 nt transcript (SL RNA) which participates in a trans-splicing process (Krause and Hirsh, Cell 49:753, 1987). This SL RNA region of the C. briggsae 1 kb 5S DNA repeat also appears to be transcribed in vivo, while the corresponding region of the 0.7 kb repeat is not.