Summary: Urea is an important nitrogen source for many microorganisms, but urea active transporters have not been characterized at a molecular level in any bacterium. Cells of Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120 exhibited the capacity to take up [¹⁴C]‐urea from low‐concentration (<1 μM) urea solutions. The K_s of Anabaena cells for urea was about 0.11 μM, and the observed uptake activity involved the transport and metabolism of urea. In contrast to urease, which was constitutively ex‐pressed, expression of the high‐affinity urea uptake activity was subjected to nitrogen control. In an Anabaena ureG (urease^–) mutant, a concentrative, active transport of urea could be demonstrated. We found that a mutant of open reading frame (ORF) sll0374 from the Synechocystis genomic sequence lacked urea transport activity. This ORF encoded a conserved component of an ABC‐type transporter, but it is not clustered together with any other possible transporter‐encoding gene. An Anabaena homologue of sll0374, urtE, was isolated and found to be part of a cluster of genes, urtABCDE, putatively encoding all the elements of an ABC‐type permease. Although the longest transcript that we could detect only covered urtABC, the impairment of urea transport by inactivation of urtA, urtB or urtE suggested that the whole gene cluster is expressed producing the urea permease. Expression was induced under nitrogen‐limiting conditions, and a complex promoter regulated by the cyanobacterial global nitrogen control transcription factor NtcA was found upstream from urtA. Our work adds urea to the known substrates of the versatile class of ABC‐type transporters and suggests the involvement of a transporter of this superfamily in urea scavenging by some bacteria in natural environments.

Ammonium is an important nitrogen source for many microorganisms and plants. Ammonium transporters whose activity can be probed with [14C]methylammonium have been described in several organisms including some cyanobacteria, and amt genes encoding ammonium/methylammonium permeases have been recently identified in yeast, Arabidopsis thaliana, and some bacteria. The unicellular cyanobacterium Synechocystis sp. PCC 6803 exhibited a [14C]methylammonium uptake activity that was inhibited by externally added ammonium. Three putative amt genes that are found in the recently published complete sequence of the chromosome of strain PCC 6803 were inactivated by insertion of antibiotic resistance-encoding gene-cassettes. The corresponding mutant strains were impaired in uptake of [14C]methylammonium. Open reading frame sll0108 (amt1) was responsible for a high affinity uptake activity (Ks for methylammonium, 2.7 microM), whereas open reading frames sll1017 (amt2) and sll0537 (amt3) made minor contributions to uptake at low substrate concentrations. Expression of the three amt genes was higher in nitrogen-starved cells than in cells incubated in the presence of a source of nitrogen (either ammonium or nitrate), but amt1 was expressed at higher levels than the other two amt genes. Transcription of amt1 was found to take place from a promoter bearing the structure of the cyanobacterial promoters activated by the nitrogen control transcription factor, NtcA.

Down's syndrome cell adhesion molecule (DSCAM) belongs to the Down's syndrome critical region of human chromosome 21, and it encodes a cell adhesion molecule involved in dendrite morphology and neuronal wiring. Although the function ofDSCAMin the adult brain is unknown, its expression pattern suggests a role in synaptic plasticity. Local mRNA translation is a key process in axonal growth, dendritogenesis, and synaptogenesis during development, and in synaptic plasticity in adulthood. Here, we report the dendritic localization ofDSCAMmRNA in the adult mouse hippocampus, where it associates with CPEB1 [cytoplasmic polyadenylation element (CPE) binding protein 1], an important regulator of mRNA transport and local translation. We identified fiveDSCAMisoforms produced by alternative polyadenylation bearing different combinations of regulatory CPE motifs. Overexpression of DSCAM in hippocampal neurons inhibited dendritic branching. Interestingly, dendritic levels ofDSCAMmRNA and protein were increased in hippocampal neurons from Ts1Cje mice, a model of Down's syndrome. Most importantly,DSCAMdendritic translation was rapidly induced by NMDA in wild-type, but not in Ts1Cje neurons. We propose that impairment of the NMDA-mediated regulation ofDSCAMtranslation may contribute to the alterations in dendritic morphology and/or synaptic plasticity in Down's syndrome.

As in other diseases associated with mental retardation, dendrite morphology and synaptic plasticity are impaired in Down's syndrome (DS). Both these features of neurons are critically influenced by BDNF, which regulates local dendritic translation through phosphatidylinositol 3-kinase-Akt-mammalian target of rapamycin (mTOR) and Ras–ERK signaling cascades. Here we show that the levels of BDNF and phosphorylated Akt–mTOR (but not Ras–ERK) pathway proteins are augmented in hippocampal dendrites of Ts1Cje mice, a DS model. Consequently, the rate of local dendritic translation is abnormally high and the modulatory effect of exogenous BDNF is lost. Interestingly, rapamycin (a Food and Drug Administration-approved drug) restores normal levels of phosphorylated Akt-mTOR proteins and normal rates of local translation in Ts1Cje neurons, opening new therapeutic perspectives for DS. The NMDAR inhibitors APV, MK-801, and memantine also restore the normal levels of phospho-mTOR in dendrites of Ts1Cje hippocampal neurons. We propose a model to explain how BDNF-mediated regulation of local translation is lost in the Ts1Cje hippocampus through the establishment of a glutamatergic positive-feedback loop. Together, these findings help elucidate the mechanisms underlying altered synaptic plasticity in DS.