Recognition of the 5'-cap structure of mRNA by eIF4E is a critical step in
the recruitment of most mRNAs to the ribosome. In Caenorhabditis elegans, s
imilar to 70% of mRNAs contain an unusual 2,2,7-trimethylguanosine cap stru
cture as a result of trans-splicing onto the 5' end of the pre-mRNA, The ch
aracterization of three eIF4E isoforms in C. elegans (IFE-1, IFE-2, and IFE
-3) was reported previously. The present study describes two more eIF4E iso
forms expressed in C. elegans, IFE-4 and IFE-5. We analyzed the requirement
of each isoform for viability by RNA interference. IFE-3, the most closely
related to mammalian eIF4E-1, binds only 7-methylguanosine caps and is ess
ential for viability. In contrast, three closely related isoforms (IFE-1, I
FE-2, and IFE-8) bind 2,2,7-trimethylguanosine caps and are partially redun
dant, but at least one functional isoform is required for viability. IFE-4,
which binds only 7-methylguanosine caps, is most closely related to an unu
sual eIF4E isoform found in plants (nCBP) and mammals (4E-HP) and is not es
sential for viability in any combination of IFE knockout. ife-2 ife-3 ife-4
, and ife-5 mRNAs are themselves trans-spliced to SL1 spliced leaders. ife-
1 mRNA is trans-spliced to an SL2 leader, indicating that its gene resides
in a downstream position of an operon.