Chlorophyllb-containing oxygenic photosynthetic prokaryotes: Oxychlorobacteria (Prochlorophytes)

The major ultrastructural and pigment characteristics of three chlorophyllb-containing photosynthetic prokaryote genera,Prochloron, Prochlorothrix, andProchlorococcus, are summarized. Aspects of their ecology are reviewed, as are also the major findings of comparisons among the three genera, and between them and the cyanobacteria, based on analyses of nucleotide sequences. The information summarized is discussed to assess how closely the three genera are related and whether they form a natural taxonomic grouping within the cyanobacteria. It is concluded that, although much of the evidence points to substantial differences among the three genera, the convenience of keeping them together as a group, namely, Oxychlorobacteria (formerly Prochlorophyta), outweighs certain inconsistencies revealed by analysis of a range of objective criteria. Current views on the possible significance of the group to considerations of the evolution of green chloroplasts are also presented.     

Amino acid accumulation in frog muscle. II. Are cycloleucine fluxes consistent with an adsorption model for concentrative uptake of amino acid?

Cycloleucine accumulation by frog muscle was studied at o °C and 25 °C. At external concentrations less than 5 mM the distribution ratio of cycloleucine is higher at 0 °C than at 25 °C. At concentrations greater than 5 mM the converse is true due to apparent exclusion of cycloleucine from a larger portion of the cell water at 0 °C.The steady state data are consistent with an absortion model for amino acid accumulation. Flux studies provide a means to rule out this model if all the possible rate-limiting steps in the movement of amino acid into and out of the cell are considered. These steps include intra-cytoplasmic diffusion, desorption from cytoplasmic or membrane sites and passage through the cell membrane. The assumption is made that the rate-limiting step for influx and efflux is the same, allowing the use of either influx or efflux data to examine the model.Diffusion-limited flux is ruled out on the basis of“influx profile analysis” of the time course of cycloleucine entry at both 0 °C and 25 °C.At least 95% of all intracellular cycloleucine leaves frog muscle cells with a single exponential time course at both 0 °C. The rate constant of efflux does not vary with cellular concentration.These findings are shown to be incompatible with desorption-limited efflux. They are compatible with membrane-limited efflex only if (i) adsorption sites are located on membranes with direct access to the extracellular space and (ii) the rate constant for desorption is equal to the rate constant of membrane-limited efflux of free amino acid. It is considered unlikely that such a coincidence would occur at both 0 °C and 25 °C. Therefore, an absorption model for cycloleucine accumulation in frog muscle appears to be untenable.     

Biochemical investigation of tissue growth: towards definition of "standard" tissue samples

This paper presents an animal model [the kangaroo], a quantitative anatomical dissection procedure, and a mathematical model [two-phase linear regression] which illustrate that body tissues grow at varying rates relative to each other. An argument is developed that biochemists interested in tissue chemical activity need to be able to sample tissue of known [predicted] growth rate. It is assumed that the ability to select, say muscle tissue samples, from any one animal at a stage of its growth where the individual selected pieces of tissue have known [predicted] low, average and high growth rates would allow comparisons to be made between the sampled tissues that may elucidate the underlying biochemical mechanisms involved in the growth process. It is asserted that to establish standards for tissue samples used in biochemical growth studies, the growth rate of the sampled tissue should be one of the criteria incorporated into the definition of what is "standard" for a tissue sample.