. . . . . . . x - . . . . . . .

ained by the isolation of the enzyme involved in the f V endonuclease with activity towards FUV-irradiat ards unirradiated DNA was isolated from a crude 1: using ammonium sulphate precipitation and ion-e ification. The FUV endonuclease was found to have ~0 and 18000. The enzyme had no absolute require ~tion mixture, but ATP was essential for maximal yme concentration and pH for endonuclease activity ~ and 7.5-7.8, respectively, when the enzyme was

Lt. Excision-repair inhibitors such as caffeine decrease, Fhe induction and elimination of pyrimidine dirt aditions for the radio-isotopic labelling of DNA iJ tblished. FUV radiation induced the formation c :nce-dependent manner. The efficiency of dimer for n in E. coli. Pyrimidine dimers could not be detected i s exposed to photoreactivating light. Incubation of ir: L-photoreactivating conditions caused elimination c ially present. The excision of pyrimidine dimers in 5 by DNA degradation. This effect was not observe h the excision-repair inhibitors caffeine and acriflavil V-induced DNA degradation was greatly reduced li,~.tlr~n ¢~naoe~tlna t h a t ,~Y,-i~i,n ~vant~ in the. v le ini tx

of pyrimidine dimers in formation was slightly greate~

in hydrolysates of irradiatec irradiated cells for 15 h undel of over 80% of the dimer., Synechocystis was accompa

erved in irradiated cells treatec acriflavin or after photoreactivation

tced in the absence of DN,~ excision events in the vicinity of the replication fork leac

vity. Measurement of DNA degradation after differen hat the excision system is saturated at 23100 dimers pe he rate of DNA degradation were reduced by exposin~ ~e of FUV (19.2 jm-2) , 24h before a challenge fluenc~ dation was observed under conditions of dark liquid

n, A.T. Natarajan, I. Csukas and A.A. van Zeeland, M R ( x University, Falmer, Brighton BN1 9QG (United King. ;ion Genetics and Chemical Mutagenesis, State Universit3 g 72, Leiden (The Netherlands)

i l u e n c e - u e

than in cell non- initialb nie, with FUV-indu, replication, suggesting that excisio to increased FUV sensitivit, fluences of FUV suggest that genome. The extent and the cells to a sub-lethal fluence (200 Jm-2) . DNA degradat holding.

20

M.R. James, A.R. Lehmann Cell Mutation Unit, Sussex dom); and Dept. of Radiation of Leiden, Wassenaarseweg

of irradiated cells to photoreact !ects of FUV radiation, even af ce of an efficient photoreactivati non-growth conditions, the cells

the activity of the photoreactivati as taken as evidence for compel an excision-repair endonuclease.

n of a delay in the loss of ph

for the presence of an excision- first step

FUV-irradiated ~ DN lysate of !

ion-exchange a molecu

~rement for activity. were 12-

assayed a decreased the acti

dimers was in Synecl~

sion

t com- [y high During st their did not ;en the further lity by

,'m was ~ay. An activity is 6308 in the

,etween in the

d time, -2.5/~g er gold nzyme. tigated. 8 were

a

',ater adiated

ruder dimers

9a- treated ivation.

DNA lead

ifferent )er

9using fluence

uid

MRC tg-

;y

:ed double-strand breaks (after 50 Gy). The inhibit, thesis as measured by [3H]thymidine incorporation il ;r ~, or UV irradiation, but caused up to a 2-fold inci ~) and 400 /~M). Similarly, lication except after DMS treatment in which cas erved. Survival experiments showed a considerable en 3AB but no significant difference after treatment witl Fhese results suggest that ADP-ribosylation of chrom :he joining step of excision repair after treatment wit ligase action gives long-lived strand breaks which aslation by the repair polymerization system, resulti 1 more UDS under certain conditions. The results al nt of ADP-ribosyl transferase in repair of UV- or ) le biological importance in human fibroblasts. Cytol iphocytes, however, do implicate ADP-ribosyl trans: omosomal aberrations by ionizing radiation. When G O human lymphocytes are X-irradiated and pc frequencies of chromosome breaks and exchanges ind

rease may be due to the inhibition of rejoining of sol ~ed strand breaks by 3AB. The presence of 3AB in th~ ation time is not more effective than the presence

X-ray-induced damage is ot ytogenetic studies with human

transferase in the production ot

)ost-treated with 3AB for 6 h, increase by a factor of 2. This some fraction of the X-ray-in- the cultures for 48 h up till the

)resence of 3AB for the first 6 h, aduced chromosomal aberrations are formed shortly aftei

; present during DNA replication (in unirradiated cells), tromatid exchanges increase in a dose-dependent manner DNA are more susceptible to induction of SCEs by 3AB by differential incorporation of BU into the cells. 3AB ot le experimental conditions, does not induce point muta- m with CHO cells.

~tsma, Department of Cell Biology and Genetics, Erasmu, edical Biological Laboratory TNO, Rijswijk (The Nether.

l i t t l e

lym~ chrom~

Wh~ the increas, duced strant fixation indicating that the X-ray-induced irradiation.

If 3AB or benzamide is the frequencies of sister-chromatic Cells with BU-containing DNA This can be demonstrated benzamide, under the same tions in the HGPRT system

21

N.G.J. Jaspers and D. Boo1 University, Rotterdam; Medical lands)

NA repair in human cells

ribosyl transferase, may be effici 5 mM 3-aminobenzamide (3AB). rease in cellular NAD content yl sulfate (DMS), 7-radiation or antly retarded the rejoining of DI~ only a small inhibition of repair lo effect on UV-induced strand br, chnique revealed no inhibition of

inhibitor had n in stationz ',rease at h

there was no effect on semic case a dela

ble enhanceme~ treatment with the oth

chromosomal p with DMS.'

could fi resulting in lar

also sugges

atid

Lted in tration ent of on (50 single- breaks .5 and ray-in- repair

(UDS) f DMS

DNA ry was oxicity tS.

Lvolved dbition w nick ~atches avolve-

of human

of

h, This r - i n -

he

after

;), aanner.

3AB. o r

muta-

~rasmus .*r-