Biomedical subjects

Karl Forchhammer

Publications and source records attributed to Karl Forchhammer.

23 records · Page 2Linked to original sources

PubMed2003

Signal transduction protein P(II) is required for NtcA-regulated gene expression during nitrogen deprivation in the cyanobacterium Synechococcus elongatus strain PCC 7942.

The transcription factor of the cyclic AMP receptor protein/FNR family, NtcA, and the P(II) signaling protein play central roles in global nitrogen control in cyanobacteria. A dependence on P(II) for NtcA-regulated transcription, however, has not been observed. In the present investigation, we examined alterations in gene expression following nitrogen deprivation in Synechococcus elongatus strain PCC 7942 and specifically the roles of NtcA and P(II). Global changes in de novo protein synthesis following combined-nitrogen deprivation were visualized by in vivo [(35)S]methionine labeling and two-dimensional polyacrylamide gel electrophoresis analysis. Nearly all proteins whose synthesis responded specifically to combined-nitrogen deprivation in wild-type cells of S. elongatus failed to respond in P(II)- and NtcA-deficient mutants. One of the proteins whose synthesis was down-regulated in a P(II)- and NtcA-dependent manner was RbcS, the small subunit of RubisCO. Quantification of its mRNA revealed that the abundance of the rbcLS transcript following combined-nitrogen deprivation rapidly declined in wild-type cells but not in P(II) and NtcA mutant cells. To investigate further the relationship between P(II) and NtcA, fusions of the promotorless luxAB reporter genes to the NtcA-regulated glnB gene were constructed and these constructs were used to transform wild-type cells and P(II)(-) and NtcA(-) mutants. Determination of bioluminescence under different growth conditions showed that NtcA represses gene expression in the presence of ammonium in a P(II)-independent manner. By contrast, NtcA-dependent activation of glnB expression following combined-nitrogen deprivation was impaired in the absence of P(II). Together, these results suggest that under conditions of combined-nitrogen deprivation, the regulation of NtcA-dependent gene expression requires the P(II) signal transduction protein.

Bacterial Proteins↗

PubMed2002

The novel protein phosphatase PphA from Synechocystis PCC 6803 controls dephosphorylation of the signalling protein PII.

The family of PII signal transduction proteins consists of one of the most highly conserved signalling proteins in nature. The cyanobacterial PII homologue transmits signals on the nitrogen and carbon status of the cells through phosphorylation of a seryl residue. Recently, we identified a protein phosphatase 2C (PP2C) homologue from the cyanobacterium Synechocystis PCC 6803, termed PphA, to be the cellular phospho-PII (PII-P) phosphatase. In this investigation, we characterized the enzymatic properties of PphA and investigated the regulation of its catalytic activity towards PII-P. PphA dephosphorylates phosphocasein and PII-P with similar efficiency in a strictly Mg2+- or Mn2+-dependent reaction. Low-molecular-weight phosphorylated molecules are poor substrates for PphA. Its reactivity towards PII-P, but not towards phosphocasein, is inhibited by various nucleotides, suggesting that this effect is based on specific properties of the PII protein. The inhibitory effect of ATP can be strongly enhanced by the addition of 2-oxoglutarate or oxaloacetate. At low concentrations of 2-oxoglutarate, changes in the ATP levels within the physiological range affect the degree of PII-Pase inhibition, whereas at 2-oxoglutarate levels beyond 0.1 mM, inhibition is almost complete at very low ATP levels. This suggests that PII dephosphorylation is not only sensitive to 2-oxoglutarate and oxaloacetate levels, it also integrates signals from the energy charge of the cells under specific cellular conditions.

Adenosine Diphosphate↗

PubMed2000

Stress responses of Synechocystis sp. strain PCC 6803 mutants impaired in genes encoding putative alternative sigma factors.

In the complete genome sequence of the cyanobacterium SYNECHOCYSTIS: sp. strain PCC 6803 [Kaneko et al. (1996 ). DNA Res 3, 109-136] genes were identified encoding putative group 3 sigma-factors SigH (Sll-0856), SigG (Slr-1545) and SigF (Slr-1564) and the regulatory protein RsbU (Slr-2031). Mutations in these genes were generated by interposon mutagenesis to study their importance in stress acclimation. For the genes sigH, sigF and rsbU, the loci segregated completely. However, attempts to mutagenize the sigG locus resulted in merodiploids. Under standard growth conditions only minor differences were detected between the mutants and wild-type. However, cells of the RsbU mutant showed a clear defect in regenerating growth after a nitrogen- and sulphur-starvation-induced stationary phase. After applying salt, heat and high-light shocks, stress protein synthesis was analysed by means of one- and two-dimensional electrophoresis. Cells of the SigF mutant showed a severe defect in the induction of salt stress proteins. Although the acclimation to moderate salt stress up to 684 mM NaCl was not significantly changed in this mutant, its ability to acclimate to higher concentrations of NaCl was reduced. Northern blot experiments showed a constitutive expression of the rsbU and sigF genes. The expression of the sigH gene was found to be stress-stimulated, particularly in heat-shocked cells, whilst that of sigG was transiently decreased under stress conditions. Possible functions of these regulatory proteins in stress acclimation of Synechocystis cells are discussed.

Bacterial Proteins↗

PubMed1998

Characterization of the glnB gene product of Nostoc punctiforme strain ATCC 29133: glnB or the PII protein may be essential.

Bacterial PII proteins, encoded by glnB genes, are central signalling molecules in nitrogen regulatory pathways and are modulated by post-translational modification in response to the cellular nitrogen status. The glnB gene was cloned from the filamentous heterocyst-forming cyanobacterium Nostoc punctiforme strain ATCC 29133 (PCC 73102) by heterologous hybridization to a Synechococcus sp. strain PCC 7942 gene fragment. Expression of the cloned gene was verified by hybridization to N. punctiforme total RNA and a single cross-reactive polypeptide was observed in immunoblots of N. punctiforme extracts probed with anti-Synechococcus 7942 PII antiserum. Modification of the purified N. punctiforme PII protein by a Synechococcus 7942 PII kinase was observed, but modified forms of PII were not detected in extracts of N. punctiforme from a variety of incubation conditions. The N. punctiforme glnB gene could not be disrupted by targeted gene replacement unless a second copy of glnB was provided in trans, suggesting that the gene or gene product is essential for growth under the conditions tested.

Amino Acid Sequence↗

PubMed1998

Nitrogen-starvation-induced chlorosis in Synechococcus PCC 7942: adaptation to long-term survival.

When deprived of essential nutrients, the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 undergoes a proteolytic degradation of the phycobiliproteins, its major light-harvesting pigments. This process is known as chlorosis. This paper presents evidence that the degradation of phycobiliproteins is part of an acclimation process in which growing cells differentiate into non-pigmented cells able to endure long periods of starvation. The time course of degradation processes differs for various photosynthetic pigments, for photosystem I and photosystem II activities and is strongly influenced by the illumination and by the experimental conditions of nutrient deprivation. Under standard experimental conditions of combined nitrogen deprivation, three phases of the differentiation process can be defined. The first phase corresponds to the well-known phycobiliprotein degradation, in phase 2 the cells lose chlorophyll a prior to entering phase 3, the fully differentiated state, in which the cells are still able to regenerate pigmentation after the addition of nitrate to the culture. An analysis of the protein synthesis patterns by two-dimensional gel electrophoresis during nitrogen starvation indicates extensive differential gene expression, suggesting the operation of tight regulatory mechanisms.

Adaptation, Physiological↗