Biomedical subjects

H Karsens

Publications and source records attributed to H Karsens.

4 recordsLinked to original sources

PubMed1997

Design of thermolabile bacteriophage repressor mutants by comparative molecular modeling.

Comparative molecular modeling was performed with repressor protein Rro of the temperate Lactococcus lactis bacteriophage r1t using the known 3D-structures of related repressors in order to obtain thermolabile derivatives of Rro. Rro residues presumed to stabilize a nonhomologous but structurally conserved hydrophobic pocket, which was shown to be important for thermostability of the Escherichia coli bacteriophage lambda repressor CI, were randomized. Of the derivatives that exhibited various temperature-sensitive phenotypes, one was shown to hold promise for both fundamental and industrial applications that require the controlled production of (heterologous) proteins in L. lactis.

Amino Acid Sequence↗

PubMed1997

Autolysis of Lactococcus lactis caused by induced overproduction of its major autolysin, AcmA.

The optical density of a culture of lactococcus lactis MG1363 was reduced more than 60% during prolonged stationary phase. Reduction in optical density (autolysis) was almost absent in a culture of an isogenic mutant containing a deletion in the major autolysin gene, acmA. An acmA mutant carrying multiple coples of a plasmid encoding AcmA lysed to a greater extent than the wild-type strain did. Intercellular action of AcmA was shown by mixing end-exponential-phase cultures of an acmA deletion mutant and a tripeptidase (pepT) deletion mutant. PepT, produced by the acmA mutant, was detected in the supernatant of the mixed culture, but no PepT was present in the culture supernatant of the acmA mutant. A plasmid was constructed in which acmA, lacking its own promoter, was placed downstream of the inducible promoter/operator region of the temperate lactococcal bacteriophage r1t. After mitomycin induction of an exponential-phase culture of L. lactis LL302 carrying this plasmid, the cells became subject to autolysis, resulting in the release of intracellular proteins.

Aminopeptidases↗

PubMed1996

Inducible gene expression mediated by a repressor-operator system isolated from Lactococcus lactis bacteriophage r1t.

A regulatory region of the temperate Lactococcus lactis bacteriophage r1t chromosome has been cloned and characterized. It encompasses the two divergently oriented genes rro, encoding the phage repressor, and tec. Both genes, of which the transcription start sites have been mapped, are preceded by consensus -35 and -10 promoter sequences. The region contains three 21 bp direct repeats with internal dyad symmetry which probably act as operators. Two of these repeats partially overlap the two promoter sequences. The distant third repeat is located within the tec coding sequence. Gel mobility shift assays demonstrated that Rro specifically binds to this sequence. To study possible transcriptional regulation of the region, a lacZ translational fusion with an open reading frame following tec was constructed. Under conditions that favour the lysogenic life cycle of r1t, beta-galactosidase activity was very low. Expression of the lacZ fusion could be induced 70-fold by the addition of mitomycin C at a concentration which promotes the switch of r1t from the lysogenic to the lytic life cycle. In non-induced cells, promoter activity was repressed by Rro, as a frameshift mutation in rro resulted in constitutive expression of the lacZ gene fusion.

Amino Acid Sequence↗

PubMed1996

Sequence analysis and molecular characterization of the temperate lactococcal bacteriophage r1t.

The temperate lactococcal bacteriophage r1t was isolated from its lysogenic host and its genome was subjected to nucleotide sequence analysis. The linear r1t genome is composed of 33,350 bp and was shown to possess 3' staggered cohesive ends. Fifty open reading frames (ORFs) were identified which are, probably, organized in a life-cycle-specific manner. Nucleotide sequence comparisons, N-terminal amino acid sequencing and functional analyses enabled the assignment of possible functions to a number of DNA sequences and ORFs. In this way, ORFs specifying regulatory proteins, proteins involved in DNA replication, structural proteins, a holin, a lysin, an integrase, and a dUTPase were putatively identified. One ORF seems to be contained within a self-splicing group I intron. In addition, the bacteriophage att site required for site-specific integration into the host chromosome was determined.

Amino Acid Sequence↗