Base-pair Transitions Research Articles

Mutations creating a new initiation point for expression of the histidine operon in Salmonella typhimurium

An operator normally controls expression of the histidine operon in Salmonella typhimurium . Deletion of this operator and a portion of the adjacent G gene in mutant hisG203 results in loss of expression of the entire operon. Mutant G203 does not revert to prototrophy, either spontaneously or following mutagen treatment. However, secondary mutants can be selected for their ability to grow on histidinol, i.e. for restored expression of the intact D gene. Of 145 secondary mutants isolated, at least 91 contain point mutations mapping in the G gene. Seventy-nine of these secondary mutants have been found to be identical; another twelve mutants are probably also the same. Each of the 79 point mutations maps in region VI of G and appears to be allelic with the type point mutant, 1306 . The secondary point mutations, in the G203 background, restore about 50% of the D enzymic activity found in wild-type repressed cells. Histidine repression control, absent in G203 secondary mutants, is restored when G203 is transduced to G203 + . The mutations, in the G203 + background, elicit a feedback hypersensitive G enzyme; this in turn leads to a cold sensitive phenotype. At 37°C the mutants grow like wild-type bacteria; at 20°C they require histidine or histidinol for growth. The experimental evidence indicates that the site of the secondary mutation is critical for expression of the operon in G203 . This site may, by base pair transition, become an initiator for transcription or for translation of messenger RNA.

Correlation between base-pair transition and complementation pattern in nitrous acid-induced ad-3B mutants of neurospora crassa

Nitrous acid-induced ad-3B mutants of Neurospora crassa were tested for revertibility after treatment with hydroxylamine (HA). Mutants which reverted after the treatment were classified as having guanine-cytosine (GC) at the mutant site. A high frequency (86.5%) of nitrous acid-induced ad-3B mutants resulting from base-pair transitions have GC at the mutant site, whereas in bacteriophage T 4 most nitrous acid-induced rII mutants have adenine-thymine (AT) at the mutant site. This apparent contradiction is discussed. In a random sampling of base-pair transition ad-3B mutants, we found among mutants with nonpolarized complementation pattern a lower incidence (2.3%) of AT at the mutant site than among mutants with polarized complementation pattern (25.0%) and among noncomplementing mutants (55.0%). Nonsense mutants, which specify the RNA triplets UAA, UAG, or UGA, and presumably the DNA anticodons ATT, ATC or ACT, should not be able to revert by a GC → AT transition, because by reaction with hydroxylamine the two C-containing triplets would be transformed to ATT, a nonsense triplet. The difference in the frequency of the GC pairs at mutant sites among the three different classes of complementation response indicates that nonsense mutations occur more frequently among mutants with polarized complementation patterns and among noncomplementing mutants than among mutants with nonpolarized complementation patterns.

Mutagenicity of methylhydroxylamines in Neurospora crassa

The mutagenicity of hydroxylamine (HA) and 2 hydroxylamine derivatives, methoxylamine (OMHA) and N-methylhydroxylamine (NMHA), have been tested in 22 purple adenine-requiring mutants ( ad-3B) of Neurospara crassa. Fourteen of the mutants were previously classified as reverting by base-pair substitution and 8 as reverting by base-pair insertion or deletion. HA or the HA derivatives induced no reversions in the mutants that revert by a base-pair insertion or deletion. It was therefore concluded that these compounds only induce base-pair substitutions. The correlation between reversion frequencies obtained with HA and OMHA on the 14 mutants which revert by a base-pair substitution indicates that OMHA reacts like HA and induces base-pair transitions predominantly of the type guanine-cytosine to adenine-thymine. The correlation between reversion frequencies obtained with HA and NMHA on these same 14 mutants suggests that NMHA is less specific and induces both types of base-pair transitions. The use of OMHA instead of HA for characterization of the mutation direction in base-pair transitions is discussed.

Hydroxylamine as a mutagenic agent for Nuerospora Crassa

The mutagenicity of hydroxylamine (HA) has been tested in 8 different adenine-requiring mutants ( ad-3B) of Nuerospora crassa: 4 mutants of this tester set revert after treatment with nitrous acid and ethyl methanesulfonate indicating that they revert by a base-pair substitution, 2 mutants revert only after treatment with ICR-170 indicating that they revert by a base-pair insertion or deletion, and 2 mutants revert only spontaneously. HA induced reversions in 3 of the 4 mutants which revert by base-pair substitution and in none of the others. The specificity of the action of HA on the mutants in the present tester set is consistent with the hypothesis that the predominant class of genetic alterations induced by HA in Neurospora is base-pair transitions from guanine-cytosine to adenine-thymine. Furthermore, it was found that the reversion frequency after HA treatment increases in proportion to the square of the treatment time.

Induction of reverse mutations and cross reactivation of nitrous acid-treated phage T4

The ability to supply the intact rIIA or rIIB function, or contribute rII + genetic markers, has been measured for nitrous-acid inactivated phages. The results show that at most one out of about ten lethal hits can prevent the rII region from entering the bacterium. Induction of reverse mutations by nitrous acid could be demonstrated for practically all mutants that had been induced by 5-bromouracil or 2-aminopurine. A cruder test showed that mutants induced by nitrous acid also responded in this way. In contrast, a number of spontaneous mutants did not show any induced reversions. In all cases where induced reversions have been analyzed carefully, a single-hit curve has been observed. The frequency of revertants versus survivors was about that expected if the deamination of a single base can cause a mutation. The rates of induced reversion for different mutants, span a range of two powers of 10. This shows that there is a significant mutagenic site specificity in addition to the difference for the two kinds of base-pair transitions.