Spontaneous deletion formation in several classes of Escherichia coli mutants deficient in recombination ability

Interfaces of the yeast killer phenomenon

A new prophylactic and therapeutic antimicrobial strategy based on a specific physiological target that is effectively used by killer yeasts in their natural ecological competition is theorized. The natural system exploited is the yeast killer phenomenon previously adopted as an epidemiological marker for intraspecific differentiation of opportunistic yeasts, hyphomycetes, and bacteria. Pathogenic microorganisms (Candida albicans) may be susceptible to the activity of yeast killer toxins due to the presence of specific cell wall receptors. On the basis of the idiotypic network, we report that antiidiotypic antibodies, produced against a monoclonal antibody bearing the receptor-like idiotype, are in vivo protecting animals immunized through idiotypic vaccination and in vitro mimicking the antimicrobial activity of yeast killer toxins, thus acting as antibiotics.

Studies on Tn10 transposition and excision in DNA-repair mutants of Salmonella typhimurium

Transposition of Tn10 in polA, recA, uvrB, mutH and uvrD mutants of Salmonella typhimurium was studied by a mating-out assay mediated by R plasmid pKM101. A decrease in transposition frequency was observed with polA, recA and uvrD mutants; uvrB and mutH mutants showed frequencies somewhat higher than control values. No effect of dimethyl sulfoxide, sodium acetate or nitrofurazone on Tn10 transposition was observed with this assay. Precise excision of Tn10 from srl202::Tn10 in these DNA-repair mutants was also studied. An increase in excision frequency of about 20 or 150 times in 2 different polA mutants, and a smaller increase, of about 2 or 15 times over control values, was detected in mutH and uvrD mutants, respectively.

Overlapping direct repeats stimulate deletions in specially designed derivatives of plasmid pBR325 in Escherichia coli

Current misalignment mutagenesis models have identified certain sequences such as direct and inverted repeats, which can stabilize transient misalignments on single-stranded DNA, as major structural parameters for deletions. We have constructed derivatives of the plasmid pBR325 to investigate further the relative roles of such sequences under controlled conditions. The plasmid derivatives pOCE15, pRS1 and pRS4, were obtained by cloning fragments of the same approximate size (60-64 bp) but different sequence into the unique EcoR1 site of the chloramphenicol acetyl transferase (cat) gene of plasmid pBR325. The insert of pOCE15 is a perfectly palindromic lac operator fragment. Both pRS1 and pRS4 carry the same non-palindromic fragment but differ from each other in the sequence at the right (3') end of the insert. Plasmid pRS4 differs from pRS1 by a 9-bp duplication containing an additional EcoR1 site at the 3' end of the insert. This arrangement yields overlapping imperfect 17-18 bp and perfect 11 bp direct repeats at the deletion termini and creates multiple opportunities for the stabilization of misaligned intermediates. The deletion rate, measured from the reversion of chloramphenicol sensitivity (Cms) to resistance (Cmr), was always highest in pRS4, intermediate in pOCE15 and lowest in pRS1, with an approximate ratio of 100:10:1. We have also obtained evidence for the participation of RecA in the genesis of deletions in these pBR325-derived plasmids.

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

Catabolite repression in Escherichia coli K12 mutants defective in glucose transport

The phenomenon of glucose catabolite repression was studied in Escherichia coli mutants unable to transport this carbohydrate. The pts I,H mutant P34 was much less sensitive to permanent and transient repressive effect of glucose on beta-galactosidase synthesis than parental type. The 1103 mutant with lack of enzyme 1 of the phosphoenolpyruvate-dependent phosphotransferase system (ptsI) behaves as well as P34 mutant after addition of glucose to casamino acids mineral medium. But in minimal medium with succinate as the sole source of carbon cells of the 1103 mutant (in accordance with the data of Perlman and Pastan, 1969) show hightened sensibility to transient glucose repression. The effect of hypersensibility disappears when the lacI mutation rendering the beta-galactosidase synthesis to costitutivity is introduced in 1103 mutant. It is shown that the hightened sensibility of beta-galactosidase synthesis to glucose transient repression in 1103 mutant is not an effect of the pts mutation and most probably is due to "inducer exclusion" of the lac operon. It is also shown that if one introduces the P34 mutation in strain devoided of one of the enzymes II for glucose (gptA) (and due to this resistant to glucose catabolite repression) then the level of resistance in double mutant does not increase in spite of considerable supression of 14C glucose accumulation. It is discussed the role of separate components of Escherichia coli K12 glucose transport system in realization of the phenomenon of catabolite repression.

IS1 is involved in deletion formation in the gal region of E. coli K12

The DNA sequence IS1, which is 800 pairs long, has been shown to integrate into various bacterial and phage operons. The presence of this DNA sequence in the gal operon of E. coli K12 leads to an 30-2000 fold increase in deletion formation in the gal region as compared to wildtype. This high frequency of deletion formation is specific for IS1 and is independent of the cellular recA function. While the frequency of reversion of gal::IS1 mutations, which also is independent of recA, is not affected by the growth temperature of the cells, the formation of deletions in the gal::IS1 system is strongly dependent on the temperature of growth. Mapping experiments showed that one endpoint of the deletions in most cases is at the site of the IS1 mutation and the second endpoint seems to be at various but preferred sites. The formation of the different classes of delections observed is affected differently by the growth temperature of the cells. A model to account for these results is presented.

Genetic studies on plasmid-linked cadmium resistance in Staphylococcus aureus

The genetic basis of cadmium resistance conferred by three penicillinase plasmids, PI(524), PI(258), and PII(147), of Staphylococcus aureus was examined by mutation, recombination, and deletion analysis. Three separate loci were identified: cadA, responsible for high-level resistance; cadB, giving a low-level resistance, nonadditive to cadA; and mad, a locus marginally decreasing the cadmium resistance of plasmid-positive staphylococci. The loci cadA and mad were present on all three plasmids, but cadB was only found on PII(147). Spontaneous deletions of mad involved up to three-fourths of the plasmid genome, which allowed derivation of a partial deletion map of PII(147), a plasmid with a contour length of 10.9 mum, corresponding to a molecular weight of 20.4 x 10(6).

Influence of chromosome structure on the frequency of tonB trp deletions in Escherichia coli

The frequency of tonB trp deletions varies in different strains and substrains of Escherichia coli. Studies with chromosomal hybrids constructed by transducing various segments of the cysB-trp-suIII region from K-12(Ymel) into K-12(W3110) indicate that the characteristic low deletion frequency of K-12(Ymel) is determined largely by the (genetic) structure of the trp-suIII region of the chromosome. Transduction of the trp region from K-12(W3110) or K-12(Ymel) into strain B has little effect on the frequency of tonB trp deletions in that strain. When tonB trp deletions occur at 42 C rather than at 37 C, there is a significant reduction in the frequency of deletions in all strains examined except K-12(Ymel) and hybrids exhibiting a Ymel deletion pattern. The magnitude of this temperature effect in different K-12 strains increases proportionally with the frequency of tonB trp deletions at 37 C. At 42 C the frequency of tonB trp deletions in all K-12 strains approaches the low frequency observed for Ymel at 37 or 42 C. In contrast, spontaneous deletions in another region of the genome which simultaneously result in resistance to phages T7 and lambda and in proline auxotrophy (tfrA pro deletions) occur at a constant frequency regardless of growth temperature or the structure of the chromosome in the trp region. Two mutants of strain KB30 obtained after treatment with nitrosoguanidine show very low tonB trp deletion frequencies. The alterations in both mutants map in the trp region of the chromosome. These studies indicate that the structure of the cysB-trp-suIII region is responsible for many of the characteristic deletion frequencies observed.