Worm Breeder's Gazette 8(1): 46

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.

Excision of Tc1 Elements

S. Emmons, L. Yesner, K. Ruan

The excision events that Tc1 elements in Bergerac undergo at high 
frequency occur in somatic cells and not in germ cells.  We drew this 
conclusion earlier from experiments showing that Tc1 elements were 
stably inherited during long-term propagation.  We have now been able 
to demonstrate directly that excision is confined to somatic cells by 
studying the amount of excision that has occurred as a function of age 
in staged populations of worms.
We synchronized Bergerac cultures by isolating embryos from young, 
gravid hermaphrodites with hypochlorite.  These embryos were allowed 
to develop and at various stages samples were taken for DNA isolation. 
In each DNA preparation the amount of excision that had taken place 
at three Tc1 sites was assessed using flanking-sequence probes.  (We 
used pCe1001, isolated in our laboratory, pCe(Br)T1, isolated in 
Hirsh's laboratory, and pCe1003, a subclone from phage lambda 3-8 
isolated by Bill Sharrock.)  For all three probes the fraction of 
empty sites was found to steadily increase during larval development 
from as little as 0.6% in embryos to as much as 10% in late larvae.  
The fraction in adults was lower, 2-3%, as expected from growth of 
germ cells and internal embryos.  Most particularly, however, the 
level in embryos of the next generation fell once again to the lowest 
level, indicating that empty Tc1 sites were not inherited.  
Preliminary results with worms arrested by starvation suggest that 
excision occurs at a steady rate independent of developmental stage, 
since the amount of excision was found to correlate with time since 
fertilization rather than with stage.
We have been able to detect extrachromosomal copies of Tc1 that may 
be the excised elements.  When Bergerac DNA is fractionated on an 
agarose gel without treatment with a restriction endonuclease, and 
hybridized on a Southern to a Tc1-specific probe, three low molecular 
weight species can be seen with mobilities that are consistent with a 
1.6 kb linear, relaxed circle, and supercoil.  The level of 
hybridization to these species in DNA from L4 larvae corresponds to 
about one copy per cell, and appears to increase during development, 
suggesting a relationship to the excision process.  We have isolated 
these molecules on sucrose gradients and begun to analyze their 
structure with restriction endonucleases.  Results so far confirm that 
the relaxed circle and linear are free copies of Tc1.  The linear 
species has unique (not permuted) ends that correspond to the ends of 
an inserted Tc1 element.
Although failing to identify a Tc1 at the unc-22 site, our analysis 
of the recombinants has revealed two putative unc-22 linked Tc1 
polymorphisms, one between daf-14 and unc-22 (a 2 m.u.  interval), and 
one between unc-22 and dpy-4 (a 4.5 m.u.  interval).  The putative 
polymorphism to the left of unc-22 is a 2.1Kb HindIII fragment 
containing Tc1 which we have cloned into lambda 590.  At present we 
are trying to isolate a unique sequence segment from this HindIII 
fragment so that we can use it to position the polymorphism with 
greater certainty.  If the fragment is suitably close to unc-22 we may 
be able to use it to walk to the unc-22 gene.  In conclusion, we feel 
that Tc1 is unlikely to be responsible for generating the unc-22 
mutations, but linked Tc1 polymorphisms may be useful in providing us 
with a molecular probe for the region.