Contact inhibition, polyribosomes, and cell surface membranes in cultured mammalian cells

An “overlay” method for rapidly and synchronously inducing contact inhibition in normal cultured cells has been developed. Using this method, disaggregation of cytoplasmic polyribosomes has been observed to occur within a matter of hours after overlay, followed by a decrease in cellular ribosomal RNA. Polysome disaggregation was influenced by the extent of cell-cell interaction and was inhibited by pretreatment of overlay cells with cycloheximide. Treatment of underlay cells with cytosine arabinoside also induced polysome disaggregation, but only after an appreciable lag as compared to that observed in overlaid cultures. Disaggregation could be induced by this method in cultured cells derived from normal tissue but not in cells derived from cancerous tissue. Polysome synthesis in growing “normal” cells (as measured by incorporation of tracer uridine into RNA) was markedly decreased when a cell surface membrane preparation was added to cultures.     

Pigmented nanoflagellates in the coastal western subtropical Pacific are important grazers on Synechococcus populations

Although the key grazers on Synechococcus and other planktonicmarine bacteria are generally thought to be nanoflagellates(both non-pigmented and pigmented) as well as ciliates, we previouslyfound in our western subtropical Pacific coastal study sitethat ciliates exerted almost no grazing pressure. In this study,we used fluorescently labeled particles (FLP) as Synechococcus-sizedmimics to examine the contribution of pigmented (may includeautotrophic and mixotrophic spp.) versus non-pigmented (heterotrophic)nanoflagellate grazing to Synechococcus morality. During thewarm season from June to September, > 50% of the nanoflagellatepopulation was pigmented (1.8–2 x 10³ versus 1.2–1.6x 10³ cells mL^–1). Consumption, or clearance rates perpigmented nanoflagellate, varied considerably (0.50–46.90nL cell^–1 h^–1), with the highest rates in June.Raw data showed pigmented nanoflagellate grazing to accountfor 2–94% (mean 43%) of Synechococcus production fromMay to October. Pigmented nanoflagellates consumed 12.5-foldmore Synechococcus than did ciliates. This study provides thefirst evidence that pigmented nanoflagellates are key grazersof Synechococcus populations in subtropical western Pacificcoastal waters.     

Characterization of Two Cytosolic Diacylglycerol Kinase Forms

Abstract: Two forms of rat brain cytosolic diacylglycerol kinase (EC 2.7.1.107) were separated by heparin-agarose column chromatography. These forms, designated DGK-I and DGK-II, were not interconvertible as determined by rechromatography. DGK-I and DGK-II had respective molecular masses of 88 and 180 kDa, as measured by Sepharose 6B chromatography. Both forms preferred diacylglycerol over monoacylglycerol and were insensitive to R59022. DGK-II, but not DGK-I, was activated by an activator substance prepared from chicken egg yolk. DGK-II was activated by a rat brain cytosolic activator and was exclusively sensitive to 5'-AMP-mediated inactivation. Further studies revealed that these two forms had the following distinct characteristics: (a) substrate specificity, (b) inhibition by heparin, (c) sensitivity to lysine-containing polyamino acids, and (d) responses to different phospholipids. In general, DGK-II was more responsive to various inhibitors and activators, making it a prime candidate for a regulatable enzyme.