Mismatch repair in Xenopus egg extracts: DNA strand breaks act as signals rather than excision points.

In Xenopus egg extracts, DNA strand breaks (nicks) located 3' or 5' to a mismatch cause an overall 3-fold stimulation of the repair of the mismatch in circular heteroduplex DNA molecules. The increase in mismatch repair is almost entirely due to an increase in repair of the nicked strand, which is stimulated 5-fold. Repair synthesis is centered to the mismatch site, decreases symmetrically on both sides, and its position is not significantly altered by the presence of the nick. Therefore, it appears that in the Xenopus germ cells, the mismatch repair system utilizes nicks as signals for the induction and direction of mismatch repair, but not as the start or end point for excision and resynthesis.

Mismatch-specific DNA breakdown in nuclear extract from tobacco (Nicotiana tabacum) callus.

Mismatch-specific enzymatic activity was sought for in nuclei from normal and transformed plant cells originating from tobacco (Nicotiana tabacum) callus and crown gall tumor induced by Agrobacterium tumefaciens. The specific enzymatic activity was assayed with substrates derived from synthetic oligonucleotides (19-mer sequences corresponding to the human K-ras gene). Single-base changes in the middle of the sequence were the basis for creating heteroduplexes with all eight mismatches. Homo- and heteroduplexes were ligated in a size ladder and used as substrates. We detected mismatch-specific DNA breakdown and determined basic requirements for the reactions. Kinetic analysis indicates the following reactivity order of preference: C:A=C:C=C:T greater than G:T approximately A:A approximately G:A approximately G:G approximately T:T much greater than G:C. It can be said now that specific mismatch recognition and repair activities have been detected in all kingdoms of living species.

In vivo genotoxicity of nitrates and nitrites in germ cells of male mice. I. Evidence for gonadal exposure and lack of heritable effects.

Effects of nitrate (doses of 600 and 1200 mg/kg/day during 14 days) and sodium nitrite (60 and 120 mg/kg/day during 14 days) on germ cells of male mice were investigated. The mode of application was stomach intubation. The germ cell stages analysed were spermatids (for the heritable effects) and differentiating and stem-cell spermatogonia (for direct effects). A lack of heritable translocations, sperm abnormalities, as well as morphological changes, such as changes in eyes, coat colour, testes and body weight, was demonstrated in F1 males originating from treated P males. Significant effects in treated males were found with respect to: (1) sex-chromosomal univalency in the diakinesis-methaphase I stage after the treatment of stem spermatogonia (both doses of sodium nitrate and the higher dose of sodium nitrite), (2) sperm-head abnormalities after treatment of differentiating spermatogonia (the higher dose of sodium nitrate and both doses of sodium nitrite), and (3) fertility after treatment of spermatids (the higher dose of sodium nitrite). Nonmutagenic effects and possible carcinogenic potential of the tested doses are discussed.

In vivo genotoxicity of nitrates and nitrites in germ cells of male mice. II. Unscheduled DNA synthesis and sperm abnormality after treatment of spermatids.

Effects of both sodium nitrate (doses of 600 and 1200 mg/kg/day for 3 days) and sodium nitrite (doses of 60 and 120 mg/kg/day for 3 days) on spermatids of mice were investigated by measuring unscheduled DNA synthesis (UDS) 17 days after the end of treatment, and sperm-head abnormality 11 and 17 days after the end of treatment. Neither chemical induced the UDS response in early to mid spermatids (17 days). The only positive result in the sperm-head abnormality test was obtained for the dose of 120 mg/kg/day of sodium nitrite both at 11 and 17 days after treatment. The results presented are in accordance with those of our earlier experiments with the same chemicals, suggesting their nonmutagenic action on the tested germ-cell stages of male mice.