Vitamin A--a pregnancy hazard alert.

Vitamin A is essential to human health, but concerns have arisen recently regarding its potential teratogenicity. Human and animal birth defects have been associated with the use of the vitamin A analogue, isotretinoin, or Accutane, for acne treatment, although the association of such defects with vitamin A itself is unclear. The federal Food and Drug Administration is evaluating the health issues surrounding vitamin A and, together with the manufacturer, has developed restrictions and label warnings to ensure the appropriate use of Accutane. We also have evaluated these issues, with concerns about the possible teratogenicity of high vitamin A intake during pregnancy. Practitioners should be familiar with the possible hazard of excessive dosages of vitamin A and its analogues. Vitamin A daily doses of higher than 8,000 IU for pregnant woman are not necessary for good health and are not recommended. Foods high in beta-carotene can provide the necessary amounts of vitamin A and, in contrast to the synthetic analogues, their use has not been associated with vitamin A toxicity or teratogenicity in humans or animals.     

14,15N, 13C, 57Fe, and 1,2H Q-band ENDOR study of Fe-S proteins with clusters that have endogenous sulfur ligands.

The benefits of performing ENDOR experiments at higher microwave frequency are demonstrated in a Q-band (35 GHz) ENDOR investigation of a number of proteins with [nFe-mS] clusters, n = 2, 3, 4. Each protein displays several resonances in the frequency range of 0-20 MHz. In all instances, features are seen near v approximately 13 and 8 MHz that can be assigned, respectively, to "distant ENDOR" from 13C in natural-abundance (1.1%) and from 14N (the delta m1 = +/- 2 transitions); the nuclei involved in this phenomenon are remote from and have negligible hyperfine couplings to the cluster. In addition, a number of proteins show local 13C ENDOR signals with resolved hyperfine interactions; these are assigned to the beta carbons of cysteines bound to the cluster [A(13C) approximately 1.0 MHz]. Five proteins show resolved, local delta m1 = +/- 2 ENDOR signals from 14N with an isotropic hyperfine coupling, 0.4 less than or equal to A(14N) less than or equal to 1.0, similar to those seen in ESEEM studies; these most likely are associated with N-H...S hydrogen bonds to the cluster. Anabaena ferredoxin further shows a signal corresponding to A(14N) approximately 4 MHz. Quadrupole coupling constants are derived for both local and distant 14N signals. The interpretation of the data is supported by studies on 15N- and 13C-enriched ferredoxin (Fd) from Anabaena 7120, where the 15N signals can be clearly correlated with the corresponding 14N signals and where the 13C signals are strongly enhanced. Thus, the observation of 14N delta m1 = +/- 2 signals at Q-band provides a new technique for examining weak interactions with a cluster.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactive effects of salinity, nitrogen and sulphur on the organic solutes in Spartina alterniflora leaf blades

Glycinebetaine, proline, asparagine, sucrose, glucose, and dimethylsulphoniopropionate(DMSP) were the major organic solutes in Spartina alternifloraleaf blades. To investigate the physiological role(s) of thesesolutes, the effects of salinity, nitrogen, and sulphur treatmentson leaf blade solute levels were examined. Glycinebetaine wasthe major organic solute accumulated in leaf blades grown at500 mol m^–3 NaCl, although asparagine and proline alsoaccumulated when the supply of nitrogen was sufficient. Thesesolutes may play a role in osmotic adjustment. In contrast,DMSP levels either did not change or were reduced in responseto the 500 mol m^–3 NaCl treatment. Furthermore, elevatednitrogen supply decreased leaf blade DMSP levels, which wasopposite to the response of glycinebetaine, proline, and asparagine.A 1000-fold increase in external sulphate concentration hadno effect on the leaf blade levels of DMSP, glycinebetaine,proline, or asparagine. These findings suggest that the majorphysiological role of DMSP in S. alterniflora leaf blades isnot for osmotic adjustment, even under conditions of nitrogendeficit and excess sulphur. Instead, DMSP which was presentat 45—130 µmol g^–1 dry weight, may play arole as a constitutive organic osmoticum. Key words: Spartina alterniflora, dimethylsulphoniopropionate, glycinebetaine, nitrogen, salinity     

Metabolism of glucose-6-phosphate and utilization of multiple metabolites for fatty acid synthesis by plastids from developing oilseed rape embryos

The aim of this work was to investigate the partitioning of imported glucose 6-phosphate (Glc6P) to starch and fatty acids, and to CO₂ via the oxidative pentose phosphate pathway (OPPP) in plastids isolated from developing embryos of oilseed rape (Brassica napus L.). The ability of the isolated plastids to utilize concurrently supplied substrates and the effects of these substrate combinations on the Glc6P partitioning were also assessed. The relative fluxes of carbon from Glc6P to starch, fatty acids, and to CO₂ via the OPPP were close to 2∶1∶1 when Glc6P was supplied alone. Under these conditions NADPH generated via the OPPP was greater than that required by the concurrent rate of fatty acid synthesis. Fatty acid synthesis was unaffected by the presence or absence of exogenous NADH and/or NADPH and the requirement of fatty acid synthesis for reducing power is therefore met entirely by intraplastidial metabolism. When Glc6P was supplied in the presence of either pyruvate or pyruvate and acetate, the total flux from these metabolites to fatty acids was up to threefold greater than that from either Glc6P or pyruvate when they were supplied singly. In these experiments there was little competition between Glc6P and pyruvate in fatty acid synthesis and the flux to starch was unchanged. This implies that the starch and fatty acid biosynthesis pathways did not compete for the exogenously supplied ATP on which they were strongly dependent. When Glc6P and pyruvate were provided together, the NADPH generated by the OPPP pathway was less than that required by the concurrent rate of fatty acid synthesis. This suggests that the metabolism of exogenous Glc6P via the OPPP can contribute to the NADPH demand created during fatty acid synthesis but it also indicates that other intraplastidial sources of reducing power must be available under the in-vitro conditions used.     

New telomere formation coupled with site-specific chromosome breakage in Tetrahymena thermophila.

Programmed chromosome breakage occurs in many ciliated protozoa and is accompanied by efficient new telomere formation. In this study, we have investigated the relationship between programmed chromosome breakage and telomere formation in Tetrahymena thermophila. Using specially constructed DNA clones containing the breakage signal Cbs in transformation studies, we have determined the locations of telomere addition around the breakage sites. They occur at variable positions, over 90% of which are within a small region (less than 30 bp) starting 4 bp from Cbs. This distribution is independent of the nucleotide sequence in the region or of the orientation of Cbs. In five of six cases determined, these sites occur at or before a T, and in the remaining case, the site occurs at or before a G. When sequences devoid of G or T are placed in this region, telomere addition still occurs within the region to maintain a similar distance relationship with Cbs. This efficient and healing process appears to be associated specifically with Cbs-directed breakage, since it does not occur when DNA ends are generated by restriction enzyme digestion. These results suggest a strong mechanistic link between chromosome breakage and telomere formation.     

鹅掌楸属植物的多糖壁前体和花粉管的生长

本文观察描述了中国鹅掌楸（Ｌｉｒｉｏｄｅｎｄｒｏｎｃｈｉｎｅｎｓｅ）和北美鹅掌楸（Ｌ．ｔｕｌｉｐｉｆｅｒａ）花粉在异已柱头萌发和花粉管生长期间多糖壁前体的发生、形态结构和生理功能．１、多糖壁前体在形态上有Ｐ－粒子（Ｐｏｌｙｓａｃｃｈａｒｉｄｅｐａｒｔｉｃｌｅｓ）,被膜小泡（ｃｏａｔｅｄｖｅｓｉｃｌｅ）和小泡（ｖｅｓｉｃｌｅ）三种。２、Ｐ－粒子于单核花粉期已经发生,至花粉管延伸期为发生高峰。多糖壁前体是在高尔基体,内质网和线粒体的相继、连续作用下,由淀粉质体、蛋白体和脂滴降解形成．３、Ｐ－粒子的形态随不同发育时期而变化,早期为成群的电子透明小泡,或为蛋白质束缚的挤压成多面体形,后期为内含颗粒或微纤丝的无被膜粒子或具刺被膜粒子。４、Ｐ－粒子移至管端．或融合或单个通过周质内质网（ＣＥＲ）,释放内容物参与管端壁的形成,被膜小池和小泡移至花粉管次顶端区向质膜外分泌,参与花粉管壁内层的形成,或移至管端,提供膜片。最后讨论了亲和性与超微结构特征的关系．     

肿瘤抑制基因APCmRNA在豚鼠脑中的分布

APC基因是1991年被发现的一类肿瘤抑制基因,它被定位于人第5号染色体5q21处。APC基因如发生缺失或突变,则易患直肠肿瘤,并伴有部分先天痴呆的病例。本工作在孟帆已获得的APC基因在豚鼠中的同源cDNA的基础上,完成了对它的亚克隆,并利用原位杂交和RNA酶保护分析的方法,对它在脑中的分布进行了研究。发现APCmRNA主要在海马、大脑和小脑中表达;嗅球中杂交信号稍弱,脑干中最弱。海马中阳性细胞主要是锥体细胞,小脑中则主要是内层颗粒细胞。在一个月大的豚鼠胚胎的脑中也观察到相似的表达型式。进一步的研究有助于我们更好地了解神经发育和先天痴呆发生的分子机制。     

水稻体细胞无性系R_1、R_2代中的雄性育性变异观察

通过水稻幼穗培养,１９９１－１９９２两年间,在５个品种（珍汕９７Ｂ、红源Ａ、包源Ａ、Ｗ６１５４ｓ,和南广占）中共获得了５０株雄性不育变异株,其中Ｒ_１代有４８株,Ｒ_２代有２株。在Ｒ１代,共获得５２６８株再生植株,雄性不育变异的平均频率为０．９１％（０．８３－１．０８％）;在Ｒ_２代（珍汕９７Ｂ）发生雄性不育变异的频率为２％。本文报道了多种花粉败育类型之间可以相互转变现象,此外不育和可育之间亦可以相互转变。对离体培养产生的雄性不育变异株用一批现有ＣＭＳ（Ｃｙｔｏｐｌａｓｍｉｃｍａｌｅｓｔｅｒｉｌｅ）不育系的典型保持系、恢复系进行测交,结果表明,Ｗ６１５４ｓ产生的雄性育性变异株仍保持核不育的特性;红源Ａ产生的雄性育性变异株有的可能是嵌合体,有的其败育花粉类型虽发生了变化,但其恢保关系并没有改变,有的则可能已转成类似ＷＡ型的不育材料;南广占产生的典败变异株,其恢保关系类似ＷＡ型,可能属核不育转成ＣＭＳ的首例发现。     

用蛋白质工程方法改变葡萄糖异构酶最适pH和最适温度

用寡核苷酸诱导的定点突变方法构建了葡萄糖异构酶基因的突变体（Ｎ１８４Ｄ和Ａ１９８Ｃ）。含突变体的重组质粒ｐＴＫＤ－ＧＩ１（Ｎ１８４Ｄ）和ｐＴＫＤ－ＧＩ２（Ａ１９８ｃ）在Ｅ．ｃｏｌｉＫ３８菌株中表达,用ＤＥＡＥ－Ｓｅｐｈａｒｏｓｅ ＦＦ和Ｓｅｐｈａｃｒｙｌ Ｓ－３００ＨＲ柱层析分离纯化突变酶。与野生型葡萄糖异构酶比较实验表明：（１）突变酶Ｎ１８４Ｄ的最适ｐＨ值下降了１个单位;等电点下降了０．６个单位     

中国典型草原放牧草场蝗虫为害损失及经济阈值的估算(英语)

传统上估算蝗虫在放牧草场为害损失的方法几乎都是用来测定对牧草秋季产量的影响,而实际上,在估算放牧草场蝗虫为害损失及经济阈值时,牧草的现存量而非秋季产量是更应考虑的因素.本文提出了一种适合测定蝗虫对牧草现存生物量影响的新方法,即野外挂笼饲养与蝗虫种群动态相结合的估算方法,并在此基础上组建了放牧草场蝗虫种群经济阈值模型;α_0 α_1M_1_α_2M_2_ α_3S_1 α_4S_2=C其中,M_1:狭翅雏蝗生物量;M_2:宽须蚁蝗生物量;S_1:狭翅雏蝗平均个体重量,S_2:宽须蚁蝗平均个体重量;α_0-α_4:常数.同时引入蝗虫种群数量和生物量两项参数来表达蝗虫种群的发生程度.