Evidence for adenylate nucleotide transport (ATP-ADP translocation) in vesicles of Frankia sp. strain EAN1pec

Response of respiration and nitrogenase activity in Datisca glomerata (Presl.) Baill. to changes in pO2

Respiration and nitrogenase activity by root systems of Datisca glomerata (Presl.) Baill. decline rapidly after exposure to C2H2. We measured C2H2 reduction and CO2 evolution to determine whether this decline is due to a variable barrier to O2 diffusion and to examine the nature of O2 regulation in these nodules. Changes in pO2 between 16 and 30 kPa during the decline had only small effects on respiration and nitrogenase activity. Thus the decline is not due to an increase in resistance to O2 diffusion. In contrast, in peas (Pisum sativum L.) nitrogenase activity was strongly reduced when the pO2 was lowered from 20 to 16 kPa during the decline. In Datisca there was little difference in nitrogenase activity when measurements in He-O2 mixtures were compared with Ar-O2 mixtures, showing that there is no gas phase limitation on diffusion. Therefore the major diffusion resistance in Datisca nodules is in the infected cells. We propose that this resistance is in the vesicles of Frankia and that the vesicles represent a series of compartments that vary in the degree to which nitrogenase is protected from O2. As pO2 increases or decreases, various compartments turn on and off keeping the overall rate of nitrogenase activity and respiration nearly constant.Key words: actinorhizal plants, Datisca glomerata, Frankia, nitrogenase, oxygen protection, Pisum sativum.

Anion-exchange mechanisms in bacteria

This article discusses the physiological, biochemical, and molecular properties of bacterial anion-exchange reactions, with a particular focus on a family of phosphate (Pi)-linked antiporters that accept as their primary substrates sugar phosphates such as glucose 6-phosphate (G6P), mannose 6-phosphate, or glycerol 3-phosphate. Pi-linked antiporters may be found in both gram-positive and gram-negative cells. As their name suggests, these exchange proteins accept both inorganic and organic phosphates, but the two classes of substrate interact very differently with the protein. Thus, Pi is always accepted with a relatively low affinity, and when it participates in exchange, it is always taken as the monovalent anion. By contrast, when the high-affinity organic phosphates are used, these same systems fail to discriminate between monovalent and divalent forms. Tests of heterologous exchange (e.g., Pi: G6P) indicate that these proteins have a bifunctional active site that accepts a pair of negative charges, whether as two monovalent anions or as a single divalent anion. For this reason, exchange stoichiometry moves between limits of 2:1 and 2:2, according to the ratio of mono- and divalent substrates at either membrane surface. Since G6P has a pK2 within the physiological range (pK of 6.1), this predicts a novel reaction sequence in vivo because internal pH is more alkaline than external pH. Accordingly, one expects an asymmetric exchange as two monovalent G6P anions from the relatively acidic exterior move against a single divalent G6P from the alkaline interior. In this way an otherwise futile self-exchange of G6P can be biased towards a net inward flux driven (indirectly) by the pH gradient. Despite the biochemical complexity exhibited by Pi-linked antiporters, they resemble all other secondary carriers at a molecular level and show a likely topology in which two sets of six transmembrane alpha-helices are connected by a central hydrophilic loop. Speculations on the derivation of this common form suggest a limited number of structural models to accommodate such proteins. Three such models are presented.

Nucleotide transport in Rhodobacter capsulatus

Cytoplasmic membrane vesicles isolated from the gram-negative photosynthetic bacterium Rhodobacter capsulatus catalyzed the transport of nucleotides. No transport occurred in the intact bacteria unless they were pretreated with EDTA. The transport rate was measured by incorporation of radioactive phosphate into externally added ADP or by incorporation of nonradioactive phosphate into added labeled ADP. The catalytic activities which utilized the added ADP were photosynthetic ATP synthesis, Pi-ADP exchange, and adenylate kinase. These activities were shown to occur on the cytoplasmic side of the internal membrane. The products were found in the outer medium. The rate of nucleotide transport across the membranes was comparable to the rate of photophosphorylation. These results indicated that nucleotides can be transported across the cytoplasmic membrane but not across the outer membrane of the native R. capsulatus cell. Therefore, by analogy to the mitochondrial ATP-ADP translocator, the exchange might function as an energy transfer system to the periplasm of these bacteria.