MOST MESSENGER-RNAS IN THE NEMATODE ASCARIS-LUMBRICOIDES ARE TRANS-SPLICED - A ROLE FOR SPLICED LEADER ADDITION IN TRANSLATIONAL EFFICIENCY

Some pre-mRNAs in nematodes are processed by trans-splicing. In this r eaction, a 22-nt 5' terminal exon (the Spliced leader, SL) and its ass ociated 2,2,7-trimethylguanosine cap are acquired from a specialized S m snRNP, the SL RNP. Although it has been evident for many years that not all nematode mRNAs contain the SL sequence, the prevalence of tran s-spliced mRNAs has, with the exception of Caenorhabditis elegans, not been determined. To address this question in an organism amenable to biochemical analysis, we have prepared a message-dependent protein syn thesis system from developing embryos of the parasitic nematode, Ascar is lumbricoides. Using this system, we have used both hybrid-arrest an d hybrid-selection approaches to show that the vast majority (80-90%) of A. lumbricoides mRNAs contain the SL sequence and therefore are pro cessed by transsplicing. Furthermore, to examine the effect of SL addi tion on translation, we have measured levels of protein synthesis in e xtracts programmed with a variety of synthetic mRNAs. We find that the SL sequence itself and its associated hypermethylated cap functionall y collaborate to enhance translational efficiency, presumably at the l evel of initiation of protein synthesis. These results indicate that t rans-splicing plays a larger role in nematode gene expression than pre viously suspected.