Here, we describe for the first time the Crc (catabolite repression control) protein from the soil bacterium Acinetobacter baylyi. Expression of A. baylyi crc varied according to the growth conditions. A strain with a disrupted crc gene showed the same growth as the wild type on a number of carbon sources. Carbon catabolite repression by acetate and succinate of protocatechuate 3,4-dioxygenase, the key enzyme of protocatechuate breakdown, was strongly reduced in the crc strain, whereas in the wild-type strain it underwent strong catabolite repression. This strong effect was not based on transcriptional regulation because the transcription pattern of the pca-qui operon (encoding protocatechuate 3,4-dioxygenase) did not reflect the derepression in the absence of Crc. pca-qui transcript abundance was slightly increased in the crc strain. Lack of Crc dramatically increased the mRNA stability of the pca-qui transcript (up to 14-fold), whereas two other transcripts (pobA and catA) remained unaffected. p-Hydroxybenzoate hydroxylase activity, encoded by pobA, was not significantly different in the absence of Crc, as protocatechuate 3,4-dioxygenase was. It is proposed that A. baylyi Crc is involved in the determination of the transcript stability of the pca-qui operon and thereby effects catabolite repression.The introduction of aromatic compounds into the central energy conservation pathways is accomplished in Acinetobacter baylyi via the -ketoadipate pathway (25). In Acinetobacter, this pathway contains two parallel branches separated in terms of enzymes, as well as regulation of expression, converting the two starting compounds protocatechuate and catechol into succinyl coenzyme A and acetyl coenzyme A (21). Protocatechuate breakdown requires six catalytic steps; the respective genes (pca genes), together with genes for one of several funneling pathways (qui genes), form a large operon (the pca-qui operon, about 14 kbp) (10). Regulation of the expression of this operon is directed from the intergenic region located upstream of this large gene cluster. Several levels of transcriptional regulation of pca-qui gene expression have been described, most of which have a negative effect and therefore serve to prevent gene expression. Only one mechanism causes induction at the otherwise weak promoter upstream of pcaI (pcaIp), namely, the activity of the regulator PcaU (22). In its absence, the pca-qui genes are expressed at a fairly high basal level. PcaU decreases this basal expression level. In the presence of the inducer protocatechuate, PcaU brings about high induction and is thus both a repressor and an activator (47, 53). PcaU is an IclR family member and binds to a site between the pca-qui genes and its own gene containing three repetitions of a 10-bp DNA sequence, which are all necessary for induction (29,36). Additional regulatory levels of higher priority can prevent induction despite the continued presence of the inducer (Fig. 1). One is a mechanism that seems to organize gene expression priorities between the two ...
High-resolution dissociative recombination rate coefficients of rotationally cool and hot H 3 + in the vibrational ground state have been measured with a 22-pole trap setup and a Penning ion source, respectively, at the ion storage-ring TSR. The experimental results are compared with theoretical calculations to explore the dependence of the rate coefficient on ion temperature and to study the contributions of different symmetries to probe the rich predicted resonance spectrum. The kinetic energy release was investigated by fragment imaging to derive internal temperatures of the stored parent ions under differing experimental conditions. A systematic experimental assessment of heating effects is performed which, together with a survey of other recent storage-ring data, suggests that the present rotationally cool rate-coefficient measurement was performed at 380 +50 −130 K and that this is the lowest rotational temperature so far realized in storage-ring rate-coefficient measurements on H 3 + . This partially supports the theoretical suggestion that temperatures higher than assumed in earlier experiments are the main cause for the large gap between the experimental and the theoretical rate coefficients. For the rotationally hot rate-coefficient measurement a temperature of below 3250 K is derived. From these higher-temperature results it is found that increasing the rotational ion temperature in the calculations cannot fully close the gap between the theoretical and the experimental rate coefficients.
We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area (10×10 cm 2 ) position-sensitive, double-sided Si-strip detector with 128 horizontal and 128 vertical strips, whose pulse height information is read out individually. The setup allows to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, as well as breakup geometries, as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the TSR storage ring facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation D2H + . A huge isotope effect is observed when comparing the relative branching ratio between the D2+H and the HD+D channel; the ratio 2B(D2+H)/B(HD+D), which is measured to be 1.27 ± 0.05 at relative electron-ion energies around 0 eV, is found to increase to 3.7 ± 0.5 at ∼ 5 eV.
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