Worm Breeder's Gazette 9(2): 45

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.

Correlation of lin-14 mRNA Levels and Structures with lin-14 Gene Activity in Mutant and Wild-Type Development

G. Ruvkun, V. Ambros, J. Giusto, and B. Horvitz

We have previously described our increasingly fine structure mapping 
of Bergerac Tc1 elements in the lin-14 gene region (Newsletter Vol.8 
No.l, No.3).  This mapping generated via a lin-14 intragenic 
recombination event a hybrid Bristol:Bergerac lin-14 gene containing 
the wild type Bergerac portion to the left and the dominant lin-14 
allele (n536) in the Bristol portion to the right, lin- 14(
Bergerac:Bristol n536).  The recombination point which generated this 
hybrid gene was mapped by detecting two EcoRI fragment length 
polymorphisms flanking this recombination point: a Bergerac-specific 1.
5 kb EcoRI fragment and a Bristol-specific 1.4 kb EcoRI fragment were 
both found in the lin-14 intragenic recombinant strain.  These data 
show that this recombination event took place between the two RFLPs 
which are located 25 kb apart on one of the cosmids we isolated from 
this region, EEG4.  Therefore cosmid EEG4 must contain the site of the 
lin-14 intragenic recombination event and must contain at least part 
of the lin-14 gene.
We used cosmid EEG4 as a probe to Southern blots of DNAs isolated 
from a total of 19 strains containing independent lin-14 alleles and 
15 non-lin-14 strains.  We have detected allele specific polymorphisms 
associated with both of the existing dominant lin-14 mutations and two 
of 17 lin-14 recessive mutations.  Three of these four detected 
alleles were induced by gamma-ray and one by EMS.  The two recessive 
alleles map to adjacent EcoRI fragments and are probably deletions.  
The two dominant alleles map about 15 kb to the right of the recessive 
alleles and to the same 6.0 kb HindIII fragment but probably do not 
overlap.  They are deletions of 600 bp (n536) and 300 bp (n355).  
These polymorphisms associated with dominant alleles are also detected 
in all recessive revertants of lin-14 dominant mutations, showing that 
the revertants still contain a cryptic dominant allele.  Using cosmid 
EEG4 as a probe, no changes in the lin-14 restriction map were 
detected in any of 15 non-lin-14 strains examined.
The cosmid EEG4 was used as a probe to RNA isolated from staged N2 
and unsynchronized cultures of a variety of lin-14 mutant strains.  
Two mRNA species were detected using this probe, one at about 7 kb and 
one at 3.5 kb.  The 3.5 kb mRNA decreased in size to about 3.0 kb in 
all strains containing the lin-14 dominant allele n536 and to about 2.
0 kb in all strains containing the lin-14 dominant allele n355.  The 7 
kb mRNA is unchanged in these strains.  In addition, in all strains 
containing the dominant alleles the level of the 3.5 kb related mRNA 
is increased about 2 to 4 fold relative to N2 (normalizing to actin-1 
mRNA), regardless of whether a recessive lin-14 allele is present in 
cis (6 out of 14 alleles have been tested).  The fact that the level 
of the 3.5 kb lin-14 mRNA is increased, for example, in both lin-14(
n536) strains showing a dominant retarded phenotype and in lin-14(
n538n536) strains showing a recessive precocious phenotype argues that 
this increase in mRNA is not a result of either retarded or precocious 
development but under a control independent of the execution of early 
or late cell lineages in the animal.  We have not analyzed all mutants 
yet, but so far none of the recessive lin-14 mutations significantly 
decreases the amount of mRNA.  Northern analysis with individual DNA 
probes from the region of the recessive mutations and from region of 
the dominant mutations showed that the 3.5 kb related mRNA is 
transcribed from both of these genomic regions which are about 12 kb 
apart.  Most likely, there is an intron(s) in this interval.
In staged wild-type worms, the amount of 3.5 kb lin-14 mRNA relative 
to actin-1 mRNA is relatively high in eggs and L1 and decreases until 
it is down about 20-fold in the L4.  The mRNA level increases again in 
adults presumably due to oogenesis.  The level of this mRNA is 0.005% 
to 0.01% of the total mRNA in unsynchronized N2.  The 7 kb mRNA 
detected with cosmid EEG4 is present at the same level relative to 
actin-1 at all stages of N2 development.  We find that the direction 
of transcription of the 3.5 kb related mRNA is oriented left to right 
on the genetic map.  Therefore the recessive mutations are located 
toward the 5' end of the gene and the dominant mutations at the 3' end.
We have not yet explained how a 3' deletion in an mRNA can cause an 
increase in the amount of that same mRNA and inappropriate activity of 
the gene product, although two attractive but speculative models come 
to mind.  In one model, there is a sequence located at the 3' end of 
the lin-14 mRNA which is involved in the degradation of the mRNA as 
larval development progresses; deletion of that sequence results in 
abnormal stability of the mRNA and associated temporally inappropriate 
lin-14 gene activity.  In the other model, there is a COOH terminal 
domain of the lin-14 gene product which is involved in either direct 
or indirect negative feedback control of lin-14 transcription; 
deletion of that domain results in loss of this negative feedback 
control and excessive late larval lin-14 gene activity.  These models 
predict very different DNA sequences in the region of the mRNA deleted 
in these dominant mutants.
These data corroborate the predictions of the genetic and 
developmental analyses of lin-14 which showed that the lin-14 gene 
product functions during early postembryonic development.  Presumably 
the lin-14 mRNA is synthesized in the egg to allow the L1 function of 
the gene product.  In addition the genetic analysis of the dominant 
mutations suggested that in normal development the level of lin-14 
gene activity decreases as the larvae develops, and that the dominant 
mutations inappropriately express a wild-type lin- 14 gene activity at 
late times in development.  Our finding that the level of lin-14 mRNA 
follows this pattern suggests that of lin-14 gene activity in both 
wild-type and dominant mutant animals is controlled at least in part 
by changing lin-14 gene transcription or mRNA stability.