Worm Breeder's Gazette 10(2): 10
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
In the last issue of the Gazette (Vol. 10, no. 1), we reported that RNAs from many species of nematodes have the same (or closely related) spliced leader as C. elegans. In C. elegans the gene encoding the SL precursor (preSL) is found on a 1 kb tandem repeat of ~110 copies that also contains the 5S rDNA (Krause and Hirsh, Cell 49:753-761, 1987). However, approximately 6 of the preSL genes are located on DNA fragments outside the 1 kb repeat. These copies are present both with and without the 5S rDNA genes. We examined the genomic organization of the preSL genes in Panegrellus redivivus and Haemonchus contortus by Southern blot analysis and screening genomic libraries. Both nematodes have many copies of the preSL gene. In P. redivivus there does not appear to be a preSL gene repeat; however, we have identified two 5S rDNA repeats, but preSL genes are not associated with these repeats. Some of the preSL genes are associated with 5S rDNA genes which are outside the repeats, while other preSL genes are independent of 5S rDNA. In H. contortus it is possible that a portion of the preSL genes are on a repeated fragment or are clustered. In contrast to C. elegans and P. redivivus, the preSL genes in H. contortus are not associated with 5S rDNA genes in repeat units or outside a repeat unit. We have isolated and sequenced eight preSL genes from genomic libraries of P. redivivus and H. contortus and compared the sequence to the C. elegans preSL gene. There are several regions of sequence conservation among the preSL genes from these three genera of worms. The 22 nt SL sequence is exactly conserved, as is the GT splice donor following the SL in seven of eight preSL genes, the three nucleotides immediately preceding the SL are conserved. It is possible that these three nucleotides are actually part of the SL but have gone undetected due to post-transcriptional modifications. In addition, an Sm binding site is found in position 65-70 in seven of eight of these genes. The gene that lacks the three conserved nucleotides preceding the SL is the same gene that lacks the Sm binding site. Therefore, we suspect that this may be a pseudogene. We are in the process of determining which of the P. redivivus and H. contortus preSL genes are expressed and what sequences outside of the preSL genes have been conserved.