Nematodes, trans- splicing

The lack of SL RNA cap recognition in nematode trans-splicing stands in apparent contrast to recent results obtained in trypanosomes. Two groups have shown that trypanosome SL RNAs, undermethylated in the cap structure, do no function in trans-splicing. These results suggest that the unique trypanosome SL RNA cap may be required at some point in the assembly of the trans-spliceosome. However, this interpretation is not clear, since it is possible that, as with other snRNAs, the cap could serve to localize the trypanosome SL RNA to the nucleus. Until the pathway of SL RNP assembly is worked out or a trypanosome cell free system is established this will remain an open question. Nevertheless, the enzymes that catalyze cap modification in... 

De Giorgi, C., De Luca, F. and Lamberti, F. (1996) A silent trans-splicing signal in the cuticlin-encoding gene of the plant-parasitic nematode Meloidogyne artiellia. Gene 170, 261-265. 

Polycistronic transcription units are typical of prokaryotic organisms and of some eukaryotic viruses. Trypanosomes (and nematodes which also trans-splice, see below) represent the only examples of eukaryotes which transcribe their genes in this way. At present, there is no evidence that the various genes encoded in polycistronic transcription units in trypanosomes are related in their function as are operons in bacteria. Likewise, we do not know what the average size of a transcription unit is in trypanosomatids and whether there is any attenuation of transcription in regions that are promoter-distal relative to those that are promoter-proximal. This latter consideration has obvious relevance for the control of gene expression in these organisms. 

Features of the Nematode SL RNA Essential for Function in Trans-Splicing... 

An homologous cell free system which utilizes synthetic SL RNA (generated by in vitro transcription) in trans-splicing has permitted a detailed dissection of the nematode SL RNA (34). Site-directed mutagenesis and chemical modification interference studies have revealed that critical functional elements in the SL RNA are confined to remarkably short regions of the molecule, all of which reside in the snRNA-like domain... 

Surprisingly, exon sequences (the 22 nt SL) are irrelevant for SL RNA function in trans-splicing in vitro (36). The fact that the 22 nt SL was not important for SL RNA participation in trans-splicing was not anticipated since this sequence has been stringently conserved in evolutionarily distant nematodes. However, this sequence conservation can be explained in part by the fact that at the DNA level, the SL sequence itself is a promoter element essential for SL RNA synthesis by RNA polymerase II (see above). 

Since nematodes carry out both c/s- and trans-splicing, it is not surprising that they contain a complete complement of U snRNAs including Ul and U5. In vitro analysis. 

As discussed above, trans-splicing serves to mature 5 ends of mRNAs embedded with polycistronic transcription units in trypanosomes and may serve the same function in nematodes. However, trans-splicing may be important in other aspects of mRNA metabolism. 

In addition to providing a discrete 5 end for mRNAs, trans-splicing also provides each mRNA with the cap structure derived from the SL RNA. Although it remains to be established whether translation in trypanosomes or nematodes is cap dependent, it seems likely that the cap, as it is in other eukaryotes, will be essential for mRNA recognition by ribosomes. It remains to be determined whether spliced leader or cap addition is necessary for transport of mRNAs from the nucleus or influences mRNA stability. 

Nilsen, T. W. (1993) Trans-splicing of nematode pre-messenger RNA Annu. Rev. Microbiol. 47 413-440. 

Maroney, P. A., Hannon, G. J., Shambaugh, J. D. and Nilsen, T. W. (1991) Intramolecular base pairing between the nematode spliced leader and its 5 splice site is not essential for trans-splicing in vitro. EMBO J. 10 3869-3875. 

Tran -splicing has been described mostly in trypanosoma, nematodes, plant/algal chloroplasts and plant mitochondria. Several cases of in vitro ftans-splicing of artificial... 

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