Worm Breeder's Gazette 14(4): 64 (October 1, 1996)

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.

Fine Structure Analysis of the C. elegans Temporal Gradient Generating LIN-14/LIN-4 RNA Duplex.

Ilho Ha, Gary Ruvkun

Dept. of Mole.Biol., Mass. General Hospital, Boston, MA 02114

The heterochronic gene lin-14 controls stage-specific cell lineages
during development by forming a temporal gradient of the LIN-14 protein.
Down-regulation of LIN-14 occurs post-transcriptionally via elements in
the 3' UTR of the lin-14 mRNA, seven of which are complementary to the
lin-4 RNAs. We propose that the lin-4 RNAs base-pair with the lin-14 3'
UTR to cause down-regulation of translation of the lin-14 mRNA.
        RNA duplex formation was demonstrated in vitro and correlated
with in vivo temporal gradient forming activity.  Mutations in the
proposed RNA duplex region of the lin-14 mRNA that disrupt in vitro RNA
duplex formation also disrupt temporal gradient formation in vivo.
These cis mutations in the lin-14 mRNA mimic the effects of a null
mutation in the regulatory lin-4 RNA.  These data show that lin-4/lin-14
RNA duplex formation is required for the generation of the LIN-14
temporal gradient.
        Many loops or bulges in RNA structures have been shown to be
important for interacting with proteins.  Four of the seven the
naturally occurring lin-4/lin-14 RNA duplexes  bulge a C residue based
on model building from sequence alignments, whereas three do not.
Reporter genes bearing multimerized (6 copies) bulged C lin-4 binding
sites down-regulate in a wild type background but not in a lin-4 mutant
background.  However similar genes bearing multimerized non-bulged lin-4
binding sites show almost no down-regulation in wild type or lin-4
mutant background.  Interestingly lin-4 RNA binds strongly in vitro  to
non-bulged lin-14 RNA binding sites but not to the bulged C lin-14 RNA
binding sites.  These results suggest that a protein factor(s) may
recognize the bulged C of lin-14/lin-4 duplex in vivo and stabilize the
lin-14/lin-4 duplex.  The bulged nucleotide may form a critical feature
for the assembly of a complex that ultimately regulates translation of
the lin-14 mRNA.  To identify such factors genetically, we have fused
the lin-14 3' UTR to an easily screenable genetic marker and are
screening for mutants which suppress the phenotype caused by
lin-14/lin-4 mediated down-regulation of the marker gene.

*I. Ha is a recipient of the postdoctoral fellowship from Cancer
Research Fund of the Damon Runyon-Walter Winchell Foundation (DRG-1287).