The influence of nucleotide excision repair (NER), the principal in vivo repair system for DNA damages, was investigated in Escherichia coli with uvrA, uvrB and uvrAuvrB mutants with the triplet repeat sequences (TRS) involved in myotonic dystrophy, the fragile X syndrome and Friedreich's ataxia. (CTG•CAG)₁₇₅ was more stable when the (CTG) strand was transcribed than when the (CAG) strand was transcribed in the alternate orientation. A lack of the UvrA protein dramatically increases the instability of this TRS in vivo as compared with the stability of the same sequence in uvrB mutant, which produces an intact UvrA protein. We propose that transcription transiently dissociates the triplet repeat complementary strands enabling the nontranscribed strand to fold into a hairpin conformation which is then sufficiently stable that replication bypasses the hairpin to give large deletions. If the TRS was not transcribed, fewer deletions were observed. Alternatively, in the uvrA⁻ mutant, the hairpins existing on the lagging strand will suffer bypass DNA synthesis to generate deleted molecules. Hence, NER, functionally similar in both prokaryotes and eukaryotes, is an important factor in the genetic instabilities of long transcribed TRS implicated in human hereditary neurological diseases.

Genetic instability investigations on three triplet repeat sequences (TRS) involved in human hereditary neurological diseases (CTG·CAG, CGG·CCG, and GAA·TTC) revealed a high frequency of small expansions or deletions in 3-base pair registers in Escherichia coli. The presence of G to A polymorphisms in the CTG·CAG sequences served as reporters for the size and location of these instabilities. For the other two repeat sequences, length determinations confirmed the conclusions found for CTG·CAG. These studies were conducted in strains deficient in methyl-directed mismatch repair or nucleotide excision repair in order to investigate the involvement of these postreplicative processes in the genetic instabilities of these TRS. The observation that small and large instabilities for (CTG·CAG)₁₇₅ fall into distinct size classes (1–8 repeats and approximate multiples of 41 repeats, respectively) leads to the conclusion that more than one DNA instability process is involved. The slippage of the complementary strands of the TRS is probably responsible for the small deletions and expansions in methyl-directed mismatch repair-deficient and nucleotide excision repair-deficient cells. A model is proposed to explain the observed instabilities via strand misalignment, incision, or excision, followed by DNA synthesis and ligation. This slippage-repair mechanism may be responsible for the small expansions in type 1 hereditary neurological diseases involving polyglutamine expansions. Furthermore, these observations may relate to the high frequency of small deletionsversus a lower frequency of large instabilities observed in lymphoblastoid cells from myotonic dystrophy patients.

Previous studies have shown that homologous recombination is a powerful mechanism for generation of massive instabilities of the myotonic dystrophy CTG·CAG sequences. However, the frequency of recombination between the CTG·CAG tracts has not been studied. Here we performed a systematic study on the frequency of recombination between these sequences using a genetic assay based on an intramolecular plasmid system in Escherichia coli. The rate of intramolecular recombination between long CTG·CAG tracts oriented as direct repeats was extraordinarily high; recombinants were found with a frequency exceeding 12%. Recombination occurred in both RecA⁺ and RecA⁻ cells but was ∼2–11 times higher in the recombination proficient strain. Long CTG·CAG tracts recombined ∼10 times more efficiently than non-repeating control sequences of similar length. The recombination frequency was 60-fold higher for a pair of (CTG·CAG)₁₆₅ tracts compared with a pair of (CTG·CAG)₁₇ sequences. The CTG·CAG sequences in orientation II (CTG repeats present on a lagging strand template) recombine ∼2–4 times more efficiently than tracts of identical length in the opposite orientation relative to the origin of replication. This orientation effect implies the involvement of DNA replication in the intramolecular recombination between CTG·CAG sequences. Thus, long CTG·CAG tracts are hot spots for genetic recombination.

The spreading mechanisms of antibiotic resistance are related to many bacterial and environment factors. The overuse of antibiotics is leading to an unceasing emergence of new multidrug resistant strains. This problem also concerns uropathogenic Escherichia coli strains, which is the most common pathogen causing urinary tract infections. The aim of this study was the genetic analysis of antibiotic resistance in comparison to the phenotypic background of E. coli strains. The characterized collection of E. coli strains isolated 10 years ago from the urine samples of patients with urinary tract infections was used for antimicrobial susceptibility testing (the disc diffusion method) and analysis of antibiotic resistance genes (PCR reaction, sequencing). Additionally, the presence of ESBL strains was analyzed. Fourteen genes were associated with resistance to beta-lactams, aminoglycosides, sulfonamides and quinolones. The genetic analysis revealed that bla_TEM-1 and sul2 were present in almost all of the studied strains. Other drug-resistance genes were very rare or non-existent. Otherwise, the phenotypic resistance to fluoroquinolones was well correlated with the genotypic background of the studied bacteria. The presence of particular genes and specific mutations indicate a high bacterial potential to multidrug resistance. On the other hand, it needs to be emphasized that the standard disk diffusion test for the routine antimicrobial susceptibility analysis is still the best way to estimate the current situation of bacterial drug-resistance.