Acyl Carrier Protein in Mycoplasmas

Acyl carrier protein (ACP) activity was determined by the malonyl-coenzyme A-CO(2) exchange reaction. It was highest in Acholeplasma laidlawii, lower in A. granularum, and lowest in A. axanthum. The sterol-requiring Mycoplasma species examined showed little or negligible ACP activity. A. laidlawii was capable of utilizing pantetheine or coenzyme A but not beta-alanine as precursor for ACP synthesis. Its ACP could thus be labeled by growing the organisms with radioactive coenzyme A. The ACP of A. laidlawii appears to be a soluble cytoplasmic protein, which could be purified about 40-fold by treatment of the cytoplasmic fluid with streptomycin sulfate and chromatography of the supernatant fluid on a Biogel P-10 column. Its molecular weight, determined by polyacrylamide gel electrophoresis, is low (about 10,900) resembling that of Escherichia coli, but it is much more sensitive to heat.     

Synthesis and biological effects of aromatic analogs of abscisic acid

Thirty six different 3-methyl-5-aryl-2,4-pentadienoic acids and esters were synthesized using the Reformatsky and Wittig reactions. The different geometrical isomers were conveniently separated by the dry column technique. Assignment of configuration of the pentadienoic side chain was based on NMR and UV properties. The biological activities of the aromatic analogs of ABA were determined in four bioassays. Most of the analogs were less active than the natural hormone. Only 3-methyl-5-p-chlorophenyl Δ²-trans, Δ⁴-trans-pentadienoic acid exhibited high ABA-like activity in all four bioassays.     

Peculiar properties of mycoplasmas: The smallest self-replicating prokaryotes

Mycoplasmas are the smallest and simplest prokaryotes capable of self-replication, with information provided by a genome which may be as small as 600 kb, estimated to carry less than 500 genes. Keeping the number of structural elements, metabolic pathways and components of the protein synthesizing machinery to an essential minimum places mycoplasmas closest to the concept of 'minimum cells'. Mycoplasmas are, therefore, most adequate candidates for the complete deciphering of the machinery of a self-replicating organism, and studies towards this goal are already underway. Living as 'minimum cells' was made possible by adopting a parasitic mode of life, securing from the host the many nutrients which cannot be synthesized by the mycoplasmas themselves. When pathogenic, infections by mycoplasmas usually follow a chronic course, with host immune reactions playing an important role in symptom production. Recent studies on the possible association of mycoplasmas with rheumatoid arthritis and AIDS are reviewed.     

Viable mouse thymocytes as a model system for studying the onset of hormone-induced cellular refractoriness

Mouse thymocytes are characterized as a model cellular system for studying the onset of hormone-induced cellular refractoriness (desensitization). This system has the following combination of useful features. (a) The cells can be isolated without the use of digestive enzymes, avoiding possible damage to surface receptors or to other exposed membranal constituents. (b) They can be kept viable for several hours, a period during which both stimulation and desensitization get well under way. (c) They can be stimulated by a variety of hormones which function via cAMP (β-agonists, prostaglandin E₁ and specific thymic humoral factors). (d) Their desensitization is receptor-specific. (e) They can be readily ruptured under mild conditions so as to allow a physiologically relevant biochemical analysis of hormonal stimulation and desensitization. (f) The hormonal response of these cells can be monitored simultaneously by the activation of adenylate cyclase, by the intracellular level of cAMP, and by the activation of cAMP-dependent protein kinase (which functions as a metabolic sensor for cAMP). In this cellular system, desensitization does not involve processes such as the efflux of cAMP, the activation of cAMP-phosphodiesterase or the synthesis of a protein mediator. On the other hand, desensitization can be accounted for by a hormone-triggered inactivation of the adenylate cyclase system. The immediate desensitization of thymocytes is reversible and occurs without apparent loss of functional receptors. Continuous presence of hormone is shown to be required not only for triggering the chain of events which leads to the readily reversible desensitization, but also for the process which transfers the cells to the subsequent, ‘locked’ desensitized state.     

Generation of selective microenvironment at the cell periphery through bulk-phase hydrophilic polymers

In order to understand the previously demonstrated effect of poly(ethylene glycol) on the stimulation of lymphocyte responses to syngeneic tumor cells (Ben-Sasson, S.A. and Henkart, P.A. (1977) J. Immunol. 119, 227–231), we have investigated the effects of addition of poly(ethylene glycol) to the medium in a number of cellular systems. The binding of trimeric IgG to tumor-lymphocyte Fc receptors was greatly enhanced by poly(ethylene glycol); a substantial increase in binding of trimeric IgG to non-Fc-receptor-bearing tumor cells was also observed. Similarly, the binding of labeled bovine serum albumin to lymphocyte surfaces was increased by poly(ethylene glycol), implying that nonspecific binding of proteins to cells was generally enhanced. The dose-response curve of concanavalin A mitogenesis was shifted to the right, as would be expected from a local increase in concanavalin A concentration. Antibody binding to erythrocytes as detected by complement lysis was similarly increased. It was found that in aqueous two-phase mixtures created by poly(ethylene glycol) and dextran, erythrocytes partition into the dextran phase through exclusion into dextran-rich microdroplets. It is proposed that addition of poly(ethylene glycol) to cell culture media creates a similar separate phase around the cell surface in which the local concentration of proteins is greater than that in the bulk medium. This concept explains many of the diverse effects of addition of poly(ethylene glycol) to the medium. It also can partially explain the requirement for serum to observe the poly(ethylene glycol) effect on the lymphocyte response to syngeneic tumor cells.     

8000 year-old human remains on the sea floor near Atlit,Israel

Our knowledge of prehistoric fishermen in the Eastern Mediterranean area is very scanty since most of the ancient coastal settlements are today inundated by the sea. The present paper describes and analyzes the human remains revealed and excavated in the 8,000 year-old submerged site at Atlit-Yam, Israel. The skeletal material indicates extensivce marine activities among the inhabitants of the village, including deep sea fishing, diving and fishnet production. The morphological similarity to the nearby Natufian population (Nahal Oren) points to population continuity in the region. The possibility of seafaring and relationships to pupulations of major mediterranean islands is also discussed.