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.

The Presence of the C. elegans Spliced Leader in other Nematodes

Susan Bektesh, Brad Rosenzweig and David Hirsh

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 
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.