Thermal inhibition of repair of methylmethane sulfonate-damaged DNA in chick embryo fibroblasts

Chick embryo fibroblasts were treated with the monofunctional alkylating agent methylmethane sulfonate at various concentrations for 1 h at 42°C, rinsed and then incubated post-treatment at various temperatures at which the kinetics of alkali-labile bond disappearance was followed. Growth experiments showed that these cells grew similarly at temperatures of either 37°C or 42°C. Repair as assessed by removal of alkali-labile bond was also similar for postincubation in the temperature range 37–42°C for damage due to methylmethane sulfonate treatment at concentrations less than 1.5 mM. When the postincubation temperature was raised higher than 42.5–43°C, this type of repair was stopped. The normal internal body temperature of adult chickens is about 41.6°C. Hence the present finding indicates that chick cells are much more severely restricted in DNA repair at temperatures above normal than are mammalian cells, which can function in this respect for several deg. C above 37°C.     

Repair of benzo[a]pyrene-initiated DNA damage in human cells requires activation of DNA polymerase alpha

Normal human fibroblasts treated with r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) yielded DNA polymerase alpha with elevated levels of activity, incorporated [3H]thymidine as a function of unscheduled DNA synthesis, and exhibited restoration of normal DNA-strand length as a function of unscheduled DNA synthesis. Lipoprotein-deficient fibroblasts treated with BPDE did not show elevated levels of DNA polymerase alpha activity, exhibited minimal [3H]thymidine incorporation, and had fragmented DNA after 24 h of repair in the absence of lipoprotein or phosphatidylinositol supplementation. When DNA polymerase beta activity was inhibited, cells with normal lipoprotein uptake exhibited [3H]thymidine incorporation into BPDE-damaged DNA but did not show an increase in DNA-strand length. DNA polymerase alpha activity and [3H]thymidine incorporation in lipoprotein-deficient fibroblasts increased to normal levels when the cells were permeabilized and low-density lipoproteins or phosphatidylinositol were introduced into the cells. DNA polymerase alpha isolated from normal human fibroblasts, but not from lipoprotein-deficient fibroblasts, showed increased specific activity after the cells were treated with BPDE. When BPDE-treated lipoprotein-deficient fibroblasts were permeabilized and 32P-ATP was introduced into the cells along with lipoproteins, 32P-labeled DNA polymerase alpha with significantly increased specific activity was isolated from the cells. These data suggest that treatment of human fibroblasts with BPDE initiates unscheduled DNA synthesis, as a function of DNA excision repair, which is correlated with increased activity of DNA polymerase alpha, and that increased DNA polymerase alpha activity may be correlated with phosphorylation of the enzyme in a reaction that is stimulated by low-density lipoprotein or by the lipoprotein component, phosphatidylinositol.     

Phospholipase C from human sperm specific for phosphoinositides

Human sperm lysates were incubated in the presence of 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphocholine, 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoethanolamine or 1-[14C]stearoyl-2-acyl-sn-glycero-3-phosphoinositol. Only the latter substrate was hydrolyzed to a significant extent, with a concomitant formation of 1-[14C]stearoyl-2-acyl-sn-glycerol. Furthermore, incubation of phosphatidyl[3H]inositol under the same conditions was accompanied by the formation, in roughly equal amounts, of [3H]inositol 1-phosphate and [3H]inositol 1:2-cyclic monophosphate. Finally [32P]phosphatidylinositol 4-phosphate and [32P]phosphatidylinositol 4,5-bisphosphate were degraded into [32P]inositol 1,4-bisphosphate and [32P]inositol 1,4,5-trisphosphate, respectively. The phosphoinositide-specific phospholipase C was activated by calcium (optimal concentration 5-10 mM) and inhibited by EGTA, although endogenous calcium supported a half-maximal activity. The enzyme displayed an optimal pH of 6.0 and an apparent Km of 0.08 mM. Its specific activity was around 10 nmol/min per mg protein, which is approximately the same as that found in human blood platelets. Subcellular fractionation revealed that 55% of the enzyme was solubilized under conditions where 80% of acrosin appeared in the supernatants. The majority of the particulate phospholipase C activity (37% of total) was found in the 1000 X g pellet, which contained only 8% of total acrosin activity. Further fractionation of spermatozoa into heads and tails indicated no specific enrichment of phospholipase C activity in any of these two fractions. However, owing to a 4-fold higher protein content in the head compared to the tail fraction, it is concluded that about 80% of particulate phospholipase C activity is located in sperm head. The physiological significance of this enzyme is discussed in relation to a possible role in acrosome reaction and (or) in egg fertilization.     

Ac transposition in transgenic tomato plants

Summary As an initial step towards developing a transposon mutagenesis system in tomato, the maize transposable element Ac was transformed into tomato plants via Agrobacterium tumefaciens. Southern analysis of leaf tissue indicated that in nine out of eleven transgenic plants, Ac excised from the T-DNA and reintegrated into new chromosomal locations. The comparison of Ac banding pattern in different leaves of the same primary transformant provided evidnece for transposition during later stages of transgenic plant development. There was no evidence of Ds mobilization in tomato transformants.     

The aluminium signal: New dimensions to mechanisms of aluminium tolerance

The physiological basis of plant reaction to and tolerance of aluminium (Al) is poorly understood. We review the results of investigations into Al toxicity and root physiology to develop a theoretical basis for explaining the reaction of the root to Al, including suggested roles for Ca²⁺, mucilaginous cap secretions and endogenous growth regulators in mediating a transmitted response between Al-damaged cap cells and the interacting cell populations of the cap and root. This information is used to identify possible mechanisms of Al tolerance, notably involving signal transduction, Al uptake pathways and root morphogenesis; and to briefly discuss how procedures selecting for Al tolerance may be improved by incorporating the concept of stimulus-response coupling. Similarities in the responses of roots to Al and other signals (e.g. gravity, light, mechanical impedance) are used to develop the hypothesis that roots respond to environmental signals by way of a common regulatory system. New research prospects for extending our perception of Al tolerance mechanisms are identified.     

Differential expression of 2′∶3′-cyclic nucleotide 3′-phosphodiesterase in cultured central,peripheral, and extraneural cells

The relative levels of the central nervous system myelin marker enzyme 2:3-cyclic nucleotide 3-phosphodiesterase (EC 3.1.4.37, CNPase) were determined in neuroblastoma, astrocyte, oligodendrocyte and Schwann cell cultures and in freshly isolated human lymphocytes and platelets. The highest specific activities were associated with the cells that elaborate myelin membrane in the central and peripheral nervous system, oligodendrocytes and Schwann cells, respectively. Antiserum to bovine CNPase recognized both CNP1 and CNP2 in CNS myelin and human oligodendroglioma. In addition, a 53,000 dalton protein was evident on autoradiographs of immunoblotted PNS myelin and human oligodendroglioma proteins. Cultured rat oligodendrocyte, C₆ and mouse NA neuroblastoma CNPase appear to share common determinants with the corresponding normal rat CNS enzyme.