用异心辅助球面法求作两曲面的交线

通过实例说明用“异心辅助球面法”求相贯线,不仅适用于回转面相交,对其中一个是非回转面,但只要曲面的截交线为圆时也可使用。既适用于两轴线相交,也适用于两曲面轴线平行的情况。     

一类周期拟环的结构

本文得到了拟环满足条件ｘｙ＝ｘｙ￣（ｎ（ｘ，ｙ）ｘ或ｘｙ＝ｙｘ￣（ｎ（ｘ，ｙ）ｙ的分解定理。     

Highly cooperative feedback control of retinal rod guanylate cyclase by calcium ions

Visual excitation in retinal rod cells is mediated by a cascade that leads to the amplified hydrolysis of cyclic GMP (cGMP) and the consequent closure of cGMP-activated cation-specific channels in the plasma membrane. Recovery of the dark state requires the resynthesis of cGMP, which is catalysed by guanylate cyclase, an axoneme-associated enzyme. The lowering of the cytosolic calcium concentration (Cai) following illumination is thought to be important in stimulating cyclase activity. This hypothesis is supported by the finding that the cGMP content of rod outer segments increases several-fold when Cai is lowered to less than 10 nM. It is evident that cGMP and Cai levels are reciprocally controlled by negative feedback. Guanylate cyclase from toad ROS is strongly stimulated when the calcium level is lowered from 10 microM to 10 nM, but only if they are excited by light. We show here that the guanylate cyclase activity of unilluminated bovine rod outer segments increases markedly (5 to 20-fold) when the calcium level is lowered from 200 nM to 50 nM. This steep dependence of guanylate cyclase activity on the calcium level in the physiological range has a Hill coefficient of 3.9. Stimulation at low calcium levels is mediated by a protein that can be released from the outer segment membranes by washing with a low salt buffer. Calcium sensitivity is partially restored by adding the soluble extract back to the washed membranes. The highly cooperative activation of guanylate cyclase by the light-induced lowering of Cai is likely to be a key event in restoring the dark current after excitation.     

A new A4 amyloid mRNA contains a domain homologous to serine proteinase inhibitors

The amyloid proteins isolated from neuritic plaques and the cerebrovasculature of Alzheimer's disease are self-aggregating moieties termed A4 protein and beta-protein, respectively. A putative A4 amyloid precursor (herein termed A4(695] has been characterized by analysis of a human brain complementary DNA. We report here the sequence of a closely related amyloid cDNA, A4(751), distinguished from A4(695) by the presence of a 168 base-pair (bp) sequence which adds 57 amino acids to, and removes one residue from, the predicted A4(695) protein. The peptide predicted from this insert is very similar to the Kunitz family of serine proteinase inhibitors. The two A4-specific messenger RNAs are differentially expressed: in a limited survey, A4(751) mRNA appears to be ubiquitous, whereas A4(695) mRNA has a restricted pattern of expression which includes cells from neuronal tissue. These data may have significant implications for understanding amyloid deposition in Alzheimer's disease.     

Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II

Ribosomal protein S6 is a component of the eukaryotic 40S ribosomal subunit that becomes phosphorylated on multiple serine residues in response to a variety of mitogens, including insulin, growth factors, and transforming proteins of many oncogenic viruses. Recently, an activated S6 kinase (S6 K II) has been purified to homogeneity from Xenopus eggs, and characterized immunologically and at the molecular level. Purified S6 K II can be deactivated in vitro by incubation with either protein phosphatase 1 or protein phosphatase 2A. Reactivation and phosphorylation of S6 K II occurs in vitro with an insulin-stimulated microtubule-associated protein-2 (MAP-2) protein kinase which is itself a phosphoprotein that can be deactivated by protein phosphatase 2A. These studies suggest that a step in insulin signalling involves sequential activation by phosphorylation of at least two serine/threonine protein kinases.     

Protease inhibitor domain encoded by an amyloid protein precursor mRNA associated with Alzheimer's disease

Amyloid B-protein/amyloid A4 is a peptide present in the neuritic plaques, neurofibrillary tangles and cerebrovascular deposits in patients with Alzheimer's disease and Down's syndrome (trisomy 21) and may be involved in the pathogenesis of Alzheimer's disease. Recent molecular genetic studies have indicated that amyloid protein is encoded as part of a larger protein by a gene on human chromosome 21 (refs 6-9). The amyloid protein precursor (APP) gene is expressed in brain and in several peripheral tissues, but the specific biochemical events leading to deposition of amyloid are not known. We have now screened complementary DNA libraries constructed from peripheral tissues to determine whether the messenger RNA encoding APP in these tissues is identical to that expressed in brain, and we identify a second APP mRNA that encodes an additional internal domain with a sequence characteristic of a Kunitz-type serine protease inhibitor. The alternative APP mRNA is present in both brain and peripheral tissues of normal individuals and those with Alzheimer's disease, but its pattern of expression differs from that of the previously reported APP mRNA.     

Resolution of quantitative traits into Mendelian factors by using a complete linkage map of restriction fragment length polymorphisms

The conflict between the Mendelian theory of particulate inheritance and the observation of continuous variation for most traits in nature was resolved in the early 1900s by the concept that quantitative traits can result from segregation of multiple genes, modified by environmental effects. Although pioneering experiments showed that linkage could occasionally be detected to such quantitative trait loci (QTLs), accurate and systematic mapping of QTLs has not been possible because the inheritance of an entire genome could not be studied with genetic markers. The use of restriction fragment length polymorphisms (RFLPs) has made such investigations possible, at least in principle. Here, we report the first use of a complete RFLP linkage map to resolve quantitative traits into discrete Mendelian factors, in an interspecific back-cross of tomato. Applying new analytical methods, we mapped at least six QTLs controlling fruit mass, four QTLs for the concentration of soluble solids and five QTLs for fruit pH. This approach is broadly applicable to the genetic dissection of quantitative inheritance of physiological, morphological and behavioural traits in any higher plant or animal.