Worm Breeder's Gazette 10(3): 157

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.

Effects of UV Radiation on Embryonic DNA Synthesis and Cell Division (Subtitle: Tough Guy Polymerases from the Worm?)

Betty-Ann Svendsen, Jennifer Reddy and Phil Hartman

Synchronous populations of young embryos were exposed to various 
fluences of 254 radiation and incubated in the presence of tritiated 
thymidine.  At selected times, samples were processed to determine the 
effects of irradiation on DNA synthesis (measured as the incorporation 
of tritium into TCA precipitable material) and cell division [measured 
by counting nuclei a la Gossett and Hecht (1980.  J.  Histochem.  
Cytochem.  28, 507-510)].  Despite the fact that over 50% of embryos 
failed to develop into adults after fluences of 25 Jm-2 or greater, 
they displayed control (unirradiated) levels of both cell division and 
replication at fluences of up to 250 Jm-2.  This resistance was not 
due to UV light attenuation, since both parameters were strongly 
inhibited in two UV radiation hypersensitive mutants (rad-1 and rad-3) 
by fluences of 10 Jm-2 and greater.  Other systems (e.g., bacteria, 
mammalian cells, yeast) are much more sensitive to the effects of UV 
radiation on DNA replication.  Analysis of the sizes of newly 
synthesized DNA in irradiated and unirradiated embryos may provide an 
explanation for this extreme radiation resistance.  Synchronous 
embryos were irradiated, pulse labeled with tritiated thymidine, and 
chased for various time intervals.  After lysis with proteinase K and 
SDS, samples were subjected to centrifugation through alkaline sucrose 
gradients.  Inclusion of [14C]-labeled T4 and T7 DNA's allowed 
determination of molecular weights.  Fluences of up to 270 Jm-2 failed 
to reduce the molecular weight of newly synthesized DNA relative to 
unirradiated controls.  This was unexpected, since damage to the 
parental strand in other organisms suppresses the size of nascent 
molecules by preventing elongation of DNA polymerase.  It can be 
calculated, based upon the induction frequency of cyclobutane dimers 
as well as the size of the newly synthesized DNA, that greater than 
ten cyclobutane dimers were induced in the template opposite each 
nascent fragment.  This leads to the provocative possibility that a 
DNA polymerase from C.  elegans is capable of trans-lesion synthesis 
through radiation-induced DNA damage.  These data are in keeping with 
the observation, described in the preceding paragraph, that DNA 
synthesis and cell division during embryogenesis are refractory to 
super-lethal doses of UV radiation.