Mutagenic effects of gamma-rays and incorporated 8-3H-purines on extracellular lambda phage: influence of mutY and mutM host mutations

Molecular analysis of point mutations in a barley genome exposed to MNU and gamma rays

We present studies aimed at determining the types and frequencies of mutations induced in the barley genome after treatment with chemical (N-methyl-N-nitrosourea, MNU) and physical (gamma rays) mutagens. We created M(2) populations of a doubled haploid line and used them for the analysis of mutations in targeted DNA sequences and over an entire barley genome using TILLING (Targeting Induced Local Lesions in Genomes) and AFLP (Amplified Fragment Length Polymorphism) technique, respectively. Based on the TILLING analysis of the total DNA sequence of 4,537,117bp in the MNU population, the average mutation density was estimated as 1/504kb. Only one nucleotide change was found after an analysis of 3,207,444bp derived from the highest dose of gamma rays applied. MNU was clearly a more efficient mutagen than gamma rays in inducing point mutations in barley. The majority (63.6%) of the MNU-induced nucleotide changes were transitions, with a similar number of G>A and C>T substitutions. The similar share of G>A and C>T transitions indicates a lack of bias in the repair of O(6)-methylguanine lesions between DNA strands. There was, however, a strong specificity of the nucleotide surrounding the O(6)-meG at the -1 position. Purines formed 81% of nucleotides observed at the -1 site. Scanning the barley genome with AFLP markers revealed ca. a three times higher level of AFLP polymorphism in MNU-treated as compared to the gamma-irradiated population. In order to check whether AFLP markers can really scan the whole barley genome for mutagen-induced polymorphism, 114 different AFLP products, were cloned and sequenced. 94% of bands were heterogenic, with some bands containing up to 8 different amplicons. The polymorphic AFLP products were characterised in terms of their similarity to the records deposited in a GenBank database. The types of sequences present in the polymorphic bands reflected the organisation of the barley genome.

Biological consequences of free radical-damaged DNA bases

The principal oxidized cytosine bases, uracil glycol, 5-hydroxycytosine, and 5-hydroxyuracil, are readily bypassed, miscode, and are thus important premutagenic lesions. Similarly the principal oxidation product of guanine, 8-oxoguanine, miscodes with A and is a premutagenic lesion. Most of the thymine and adenine products that retain their ring structure primarily pair with their cognate bases and are not potent premutagenic lesions. Although thymine glycol pairs with its cognate base and is not mutagenic it significantly distorts the DNA molecule and is a lethal lesion. Ring fragmentation, ring contraction, and ring open products of both pyrimidines and purines block DNA polymerases and are potentially lethal lesions. Although these breakdown products have the potential to mispair during translesion synthesis, the mutational spectra of prokaryotic mutants defective in the pyrimidine-specific and/or purine-specific DNA glycosylases do not reflect that expected of the breakdown products. Taken together, the data suggest that the principal biological consequences of endogenously produced and unrepaired free radical-damaged DNA bases are mutations.