FORMATION AND REPAIR OF O-6-METHYLGUANINE IN RECOMBINATION HOT-SPOTS OF PLANT CHROMOSOMES

Mutagen-induced chromatid aberrations ore not randomly distributed alo ng the metaphase chromosomes. In the field bean (Vicia faba), defined late-replicating and transcriptionally inactive heterochromatic region s ore preferentially involved. After exposure to the alkylating agent N-methyl-N-nitrosourea (MNU) (10(-3) M, 1 hour), 70% of all aberration s ore clustered within 6 segments containing tandemly repeated Fokl el ements of 59 bp, which comprise similar to 10% of the genome. Using im mune-slot-blot analyses, we have studied the frequency of O-6-methylgu anine (O-6-MeG), a mutagenic lesion important For aberration induction , in total genomic DNA as well as in Fokl sequences of the field bean after exposure to MNU. In either case, similar numbers of adducts per nucleotide were found immediately after treatment as well as after 18 hours of recovery, when most adducts were removed and significant amou nts of chromatid aberrations were detectable. Peculiarities of long Fo kl element arrays (e.g., formation of specific tertiary structures), r esulting in error-prone recombination repair, rather than preferential Formation or delayed repair of O-6-MeG are apparently responsible for aberration clustering in these hot spot regions. (C) 1997 Wiley-Liss, Inc.