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G Falk

Publications and source records attributed to G Falk.

At least 55 records · Page 3Linked to original sources
PubMed1985

Transcription of Rhodospirillum rubrum atp operon.

The photosynthetic non-sulphur bacterium Rhodospirillum rubrum contains a cluster of five genes encoding the subunits of F1-ATPase [Falk, Hampe & Walker (1985) Biochem. J. 228, 391-407]. Transcription of these genes has been studied by two methods, transcriptional mapping with S1 nuclease and primer extension analysis. Thereby a 5'-end in RNA derived from this region has been demonstrated at a guanine residue 236 bases before the initiation codon of the gene for the delta-subunit, the first in this cluster. DNA sequences on the 5' side of this nucleotide show some similarity to promoters in Escherichia coli, but are not apparently related to sequences upstream of the Rhodopseudomonas blastica atp operon. A 3'-end in RNA derived from this gene cluster has been demonstrated by S1-nuclease mapping. This is found before a run of thymidylate residues in the DNA, on the 3' side of a region of dyad symmetry. In E. coli these features are characteristic of rho-independent transcriptional termination signals. It appears from these studies and from the organization of the genes that the five genes in the atp cluster may be co-transcribed from this promoter and that transcripts terminate at the region of dyad symmetry.

Base Sequence↗
PubMed1985

Nucleotide sequence of the Rhodospirillum rubrum atp operon.

The nucleotide sequence was determined of a 8775-base-pair region of DNA cloned from the photosynthetic non-sulphur bacterium Rhodospirillum rubrum. It contains a cluster of five genes encoding F1-ATPase subunits. The genes are arranged in the same order as F1 genes in the Escherichia coli unc operon. However, as in the related organism Rhodopseudomonas blastica, neither genes for components of F0, the membrane sector of ATP synthase, nor a homologue of the E. coli uncI gene are associated with this locus, as they are in E. coli.

Amino Acid Sequence↗
PubMed1985

Dopamine hyperpolarizes and reduces the light responses of rod ON-centre bipolar cells in the retina of the dogfish, Scyliorhinus canicula.

Intracellular recordings were obtained from horizontal cells and bipolar cells of the dark-adapted, virtually all-rod retina of the dogfish. Eyecups were superfused with Ringer's solution containing micromolar concentrations of dopamine (DA). It was found that 10 microM DA hyperpolarized rod ON-centre bipolar cells and reduced their responses to light flashes. Detectable effects could be obtained with DA concentrations as low as 1 microM. A decrease in the light responses of bipolar cells due to the effects of DA was concomitant with a decrease in the amplitude of the b-wave of the electroretinogram. DA had no effect on the membrane potential or the light response of rod horizontal cells. These results demonstrate a selective action on rod ON-centre bipolar cells and suggest the existence of a dopaminergic pathway which affects the sensitivity of the rod visual system at the retinal level.

Animals↗
PubMed1984

Rhodopseudomonas blastica atp operon. Nucleotide sequence and transcription.

The nucleotide sequence has been determined of a 12,368 base-pair region of DNA cloned from the non-sulphur photosynthetic bacterium Rhodopseudomonas blastica. It contains a cluster of six genes of which five encode the subunits of F1-ATPase; the sixth codes for an unknown protein. The genes are arranged in the same order as in the Escherichia coli unc operon, except that the unknown gene is placed between those for gamma and beta subunits. Neither the genes for F0 subunits, nor a homologue of the E. coli uncI gene is associated with this locus. The six genes are transcribed from a single promoter and we have designated this region the R. blastica atp operon. The two distal genes, beta and epsilon, may also be transcribed from a second promoter. Initiation and termination points for transcription have been identified by primer extensions and S1 nuclease mapping experiments. Signals involved in initiation of translation (Shine and Dalgarno sequences) and termination of transcription in the photosynthetic bacterium resemble those in E. coli. However, no common features can be identified in these two bacteria between 5' regions adjacent to sites of initiation of transcription. The sequence also contains a gene that encodes a protein homologous to discoidin, a cell surface lectin of Dictyostelium discoideum thought to be involved in cell--cell aggregation. Seven other reading frames have not been identified.

Amino Acid Sequence↗
PubMed1982

An analysis of voltage noise in rod bipolar cells of the dogfish retina.

1. The power spectral density of voltage noise in depolarizing rod bipolar cells was analysed during darkness and steady illumination. 2. The variance of the voltage fluctuations increased nearly linearly with dim light but was suppressed by bright light. 3. The spectrum in darkness and during illumination could be resolved into two components. One component was attributed to random quantal events arising from spontaneous or light-induced isomerization of rhodopsin in the bipolar cell's rod pool. 4. The second component had a peaked spectrum and was attributed to synaptic noise. 5. A significant fraction of the noise variance in the dark arose from spontaneous thermal isomerization of rhodopsin, with a rate constant of 6 X 10(-12) s-1 or a half-life for rhodopsin of 3700 years at 17 degrees C. 6. The peak amplitude of the single-photon signal in the bipolar cell was about 200 microV, associated with a peak conductance increase of 200 pS. The spectral data suggest that there may be a random delay in the generation of these events. 7. It was concluded that the limitation to single-photon detection in the dark-adapted state may be the rate of spontaneous rhodopsin isomerization. Synaptic noise at the rod-bipolar cell level would not seriously degrade the signal-to-noise ratio.

Animals↗
PubMed1981

Photon-like signals following weak rhodopsin bleaches.

The rod visual system in its dark-adapted state behaves as a near-ideal light detector. Psychophysical studies on the reliability of light detection in man, analysis of the dark noise in rod bipolar cells and observation of photon-like events in rods in the dark suggest that the visual pigment, rhodopsin, is very stable against spontaneous isomerization. When a light which bleaches a small fraction of the rhodopsin is extinguished, the visual threshold may be increased by several orders of magnitude. The eye is then 'light-adapted' and the process (or processes) by which the sensitivity returns constitutes dark adaptation. We report here that bleaching of a small fraction of the rhodopsin produces a prolonged increase in the noise observed in the dark in rod bipolar cells of the dogfish retina. The associated noise events are similar to those produced by the absorption of light quanta and presumably have their origin in the rods which transmit their signals to the bipolar cells. This increased noise after bleaching would decrease the reliability of detection of light quanta and contribute to the elevation of visual threshold.

Animals↗
PubMed1980

Responses of rod bipolar cells in the dark-adapted retina of the dogfish, Scyliorhinus canicula.

1. Responses to light were recorded from bipolar cells in the retina of the dogfish, Scyliorhinus canicula, under dark-adapted conditions. The identity of the cells was confirmed by Procion Yellow staining.2. More than 95% of the bipolar cells sampled were of the type which depolarized to a spot of light. These are termed depolarizing bipolar cells. In most cells, illumination of the surround had little effect on the responses elicited from the central receptive field.3. The mean flash sensitivity of the depolarizing bipolar cells was 270 mV/Rh(**) (where Rh(**) signifies rhodopsin photoisomerization per rod for full field illumination).4. The mean flash sensitivity of horizontal cells under the same conditions was 8 mV/Rh(**). In a limited sample of hyperpolarizing bipolar cells the highest flash sensitivity was 42 mV/Rh(**).5. The high flash sensitivity of the depolarizing bipolar cells indicates a large voltage gain at its synapse with rods. On the assumption of a rod flash sensitivity of 2 mV/Rh(**) the mean gain at the synapse was 135, but for some cells the gain was in excess of 500.6. Responses of depolarizing bipolar cells to dim flashes could be approximated by the impulse response of a 12-16 stage low-pass filter, whereas horizontal cell responses could be fitted by a low-pass filter of six sections. The implied filter at the rod-bipolar cell synapse is tuned to the higher frequency components of rod signals, thereby improving temporal resolution in the rod pathway.7. Depolarizing bipolar cell responses to test flashes are reduced by weak background illumination (less than 0.1 Rh(**)/sec). This desensitization, which would not be expected to affect rod responses, could be explained by a shift in the operating point to a less sensitive region of the intensity-response curve as a result of the large depolarization elicited by the background.8. The results of current injection into the cell in darkness and during the response to light are consistent with the release by rod terminals of a transmitter which closes ionic channels in a conductance path having a reversal potential of - 8 mV, transmitter release being suppressed by light.

Animals↗
PubMed1980

The single-photon signal in rod bipolar cells of the dogfish retina.

1. Responses of depolarizing bipolar cells to dim light flashes were recorded with intracellular micro-electrodes in the dark-adapted retina of the dogfish, Scyliorhinus canicula. 2. Fluctuations in the responses were analysed by a method of matched filtering in order to improve the signal-to-noise ratio. 3. Both the mean and variance of the response amplitude increased linearly with light intensity for intensities not exceeding a mean of 1 photon absorbed per 50 rods. 4. On the assumption that the most significant source of the fluctuation is the quantal absorption of light by the rod outer segments, a single photoisomerization leads to a post-synaptic event of mean size 250 micronV. 5. The mean number of rods in the pool sending signals to a bipolar cell is estimated as 1600. Individual rod pools are 90-330 micrometer in diameter on the retinal surface. 6. It is estimated that the conductance of 1 divided by 400 of the total number of light-modulated ionic channels in the bipolar cell is increased by a single photon acting within its rod pool. 7. In the absence of a light stimulus, the residual noise in the output of a matched filter can be interpreted as due mainly to spontaneous isomerization of rhodopsin in the rods and behaves as the 'dark light' postulated to limit detection at absolute threshold.

Animals↗