Biomedical subjects

S Oehler

Publications and source records attributed to S Oehler.

8 recordsLinked to original sources

PubMed1995

A basic tail increases repression by dimeric lac repressor.

Tetrameric Lac repressor achieves cooperative repression by binding simultaneously to O1 and to one of the auxiliary operators O2 or O3, thereby forcing the intervening DNA into a loop. Dimeric Lac repressor is not able to form DNA loops and consequently shows no cooperative repression. We constructed a dimeric Lac repressor mutant which exhibits increased repression to the lac operon that does not depend on specific operator-repressor-operator loops. This Lac repressor carries a synthetic tail of basic residues attached to its C terminus. With this construct, we observe an increase of the in vivo repression upon addition of auxiliary lac operators to a chromosomal lac operon controlled by O1. This suggests that the basic tail enables dimeric Lac repressor to enhance its repression by additional non-specific DNA contacts.

Amino Acid Sequence↗

PubMed1994

Quality and position of the three lac operators of E. coli define efficiency of repression.

Repression of the lac promoter may be achieved in two different ways: either by interference with the action of RNA polymerase or by interference with CAP activation. We investigated cooperative repression of the Escherichia coli lac operon by systematic conversion of its three natural operators (O1, O2 and O3) on the chromosome. We find that cooperative repression by tetrameric Lac repressor increases with both quality and proximity of the interacting operators. A short distance of 92 bp allows effective repression by two very weak operators (O3, O3). The cooperativity of lac operators is discussed in terms of a local increase of repressor concentration. This increase in concentration depends on flexible DNA which allows loop formation.

Base Sequence↗

PubMed1993

Genetic analysis of the leucine heptad repeats of Lac repressor: evidence for a 4-helical bundle.

Gel-filtration experiments indicate that a peptide (P2) composed of the basic region of GCN4 fused to the leucine heptad repeats of Lac repressor forms tetrameric aggregates. Gel-shift experiments were performed to determine the orientation of the helices in the tetrameric P2 aggregate. Sandwich-complex formation of peptide P2 with two DNA fragments containing two symmetrical CRE binding sites (5'-ATGACGTCAT-3') at a distance of 21 bp suggests antiparallel aggregation of the Lac leucine heptad repeats. Thus, we conclude that the leucine heptad repeats of Lac repressor have the ability to form homomeric 4-helical bundles with an antiparallel arrangement of the helices. This topology enables the two DNA fragments in the sandwich complexes to be held together by two tetramers of peptide P2. Replacement of the uncharged amino acids of the helical g and e positions of peptide P2 by the corresponding charged residues of GCN4 (peptide P4) results in a dimeric and parallel aggregation of the leucine heptad repeats, and consequently abolishes the potential to form sandwich structures. Similarly, a hybrid Lac repressor in which the GCN4 leucine zipper replaces the natural Lac leucine heptad repeats forms dimers only. It regains the ability to form tetramers when the charged amino acids in helical positions g and e are replaced by uncharged alanines.

Amino Acid Sequence↗

PubMed1991

Dimer-to-tetramer assembly of Lac repressor involves a leucine heptad repeat.

The C-terminus of Lac repressor is responsible for the formation of repressor tetramers from active dimers. If properly grafted, the C-terminus of Lac repressor (amino acids 331 to 360) converts Gal repressor dimers into tetramers. Amino acids 342 to 356 of Lac repressor contain a 4-3 hydrophobic repeat of four leucines and one valine. Systematic amino acid replacements of all residues in this region show that the protein-protein interaction between repressor dimers depends mainly on the hydrophobic residues of the 4-3 repeat, which is constitutive for coiled coils. Thus the tetramerization site of Lac repressor resembles the leucine zipper motif found in a family of eukaryotic transcription factors.

Amino Acid Sequence↗

PubMed1990

The three operators of the lac operon cooperate in repression.

We tested the effect of systematic destruction of all three lac operators of the chromosomal lac operon of Escherichia coli on repression by Lac repressor. Absence of just one 'pseudo-operator' O2 or O3 decreases repression by wild-type tetrameric Lac repressor approximately 2- to 3-fold; absence of both 'pseudo-operators' decreases repression greater than 50-fold. O1 alone represses under these conditions only approximately 20-fold. Dimeric active Lac repressor (iadi) represses the wild-type lac operon to about the same low extent. This indicates that cooperative interaction between lac operators is due to DNA loop formation mediated by tetrameric Lac repressor. Under conditions where loop formation is impossible, occupation of O3 but not of O2 may lead to weak repression. This suggests that under these conditions CAP activation may be inhibited and that stopping transcription at O2 does not significantly contribute to repression.

Base Sequence↗

PubMed1988

Recognition helices of lac and lambda repressor are oriented in opposite directions and recognize similar DNA sequences.

Exchanges in positions 1 and 2 of the putative recognition helix allow lac repressor to bind to ideal lac operator variants in which base pair 4 has been replaced. We show here that an Arg-22----Asn exchange in position 6 of the putative recognition helix of lac repressor abolishes lac repressor binding to ideal lac operator. This lac repressor variant, however, binds to a variant of the ideal lac operator 5' TTTGAGCGCTCAAA 3' in which the original G.C of position 6 has been replaced by T.A. This result and our previous data confirm our suggestion that the N terminus of the recognition helix of lac repressor enters the major groove close to the center of symmetry of lac operator and that its C terminus leaves the major groove further away from the center of symmetry. The consequences of this model are discussed in regard to various phage and bacterial repressor operator systems.

Base Sequence↗

PubMed1985

In vitro reconstitution of messenger ribonucleoprotein particles from globin messenger RNA and cytosol proteins.

Deproteinized globin poly(A) + mRNAs reassociate readily in vitro with soluble RNA-binding proteins of the cytosol; reconstituted messenger ribonucleoprotein complexes are obtained which are very similar to native globin polyribosomal-mRNP as far as bouyant density in Cs2SO4 and the composition of proteins which can be crosslinked to the mRNA are concerned. Proteins thus identified bind specifically to mRNA and not to ribosomal RNA or any synthetic oligonucleotides, with one exception: a 78-kDa protein could be cross-linked to poly(A).

Animals↗

PubMed1977

[Behavior of the body weight in 2261 maturity-onset diabetics under conditions of ambulatory care].

Of 2,261 maturity-onset-diabetics (970 males, 1,691 females) body weight, size and Broca-index were established at the beginning of the treatment, one year after the beginning of the treatment and at the date of analysis (x=5.7 years after the manifestation of diabetes). 72% of the females and 49% of the males had a Broca-index of more than 10 at the beginning of the treatment. The average loss of weight of the diabetics of adult type with overweight with a Broca-index of more than 5 was in the first year 7.6 kg in males and 5.2 kg in females. In the first year after beginning of the disease in the diet group and in the buformin group was achieved an average reduction of weight in males of about 7 kg and in females of about 6 kg. After the first year of treatment in the groups mentioned the body weight remained or rose to one kg. The patients treated with sulfonylureas--compared with the diet-biguanide group--showed in the first year of treatment, indeed, a loss of weight which was about 50% smaller, however, also in the following years (x=4.7) a further continuous reduction of weight could be recognized. The initial results and the late results in the treatment of elderly diabetics with overweight show that, compared with former years, partial success is to be achieved with reduction cures performed in outpatient departments. The importance of the reduction of weight in elderly diabetics for the improvement of the carbohydrate tolerance and the fat metabolism is emphasized.

Adult↗