Host-specific nodulation is encoded on a 14kb DNA fragment in Rhizobium trifolii

Summary The Rhizobium trifolii genes necessary for nodule induction and development have been isolated on a 14.0kb fragment of symbiotic (Sym) plasmid DNA. When cloned into a broad-host-range plasmid vector, these sequences confer a clover nodulation phenotype on a derivative of R. trifolii which has been cured of its endogenous Sym plasmid. Furthermore, these sequences encode both host specificity and nodulation functions since they confer the ability to recognize and nodulate clover plants on Agrobacterium and a fast-growing cowpea Rhizobium. This indicates that the bacterial genes essential for the initial, highly-specific interaction with plants are closely linked.     

Crystal structure of mung bean inhibitor lysine active fragment complex with bovine beta-trypsin at 1.8A resolution.

The crystal structure of the complex of mung bean inhibitor lysine active fragment with bovine beta-trypsin has been determined by X-ray crystallographic analysis at a resolution of 1.8 A. Refinement of the model of the complex converged at a final R value of 0.16. From the resulting electron density map, about one-third of the residues of the inhibitor were identified and two residues, at position P4 and P2' respectively, were found to be inconsistent with the sequence reported previously. The peptide chain of the inhibitor at the trypsin active site turns back sharply at Pro23I and forms a 9-residue reactive loop, which interacts with trypsin in a similar manner to the other families of inhibitors, suggesting an important and common role of these regions in exhibiting inhibitory activity.     

Specification of neuronal identity in the embryonic CNS

One of the earliest and most crucial steps in the development of connectivity within the CNS is the acquisition of specific identities by developing neural cells. In this review, we discuss how a neural cell may come to acquire its unique identity and some of the genes that may be involved in this process. Experimental evidence suggests that ectodermal cells may pass through several phases at which their potential fates become progressively more restricted. An initial step occurs during neural induction when ectodermal cells become restricted to either a neural or non-neural fate. A little later in development, a further set of interactions determine which of the neural cells become postmitotic and begin a programme of differentiation. The differentiation phase may itself involve several steps at which the postmitotic neuron progressively advances towards its final identity.     

A cloned fragment of HeLa DNA containing consensus sequences of satellite II and III DNA hybridizes with the Drosophila P-element and with the 1.8 kb family of human KpnI fragments

We have cloned a repetitive EcoRI fragment from the human genome which displays weak homologies with the Drosophila melanogaster transposable P-element. This cloned DNA appeared not to be a mobile element but, instead, a divergent member of human satellite II or III DNAs. We present here the first complete nucleotide sequence of a 1.797 kilobase pair (kb) satellite-like DNA. Moreover, this EcoRI satellite monomer contains a unique sequence of 49 basepairs (bp) that is devoid of the satellite consensus repeat 5'TTCCA3'. Southern hybridization analysis revealed that the cloned insert is closely related to a highly repetitive 1.8 kb KpnI family of tandemly organized satellite DNAs. Thus, the relationships among these satellite DNA families appear to be complex and may be a factor in their copy number, position and spatial organization.     

Organization of a repetitive human 1.8 kb KpnI sequence localized in the heterochromatin of chromosome 15

We have isolated a repetitive 1.8 kb Kpnl DNA sequence which is amplified in the homogeneously staining regions of a human melanoma cell line. Under low stringency conditions this sequence (D15Z1) hybridized in situ to the centromeric heterochromatin of chromosomes 1, 9, 15p, 16, and distal Yq as well as to the the short arms of the other acrocentric chromosomes. Under conditions of high stringency, labelling was predominantly on the short arm of chromosome 15. D15Z1 was shown to be present at approximately 3,000 copies per haploid genome and organized in long tandem arrays showing restriction site heterogeneity. Sequences homologous to D15Z1 were highly enriched in the less dense shoulder region of a Ag⁺—Cs₂SO₄ gradient. Analysis of D15Z1 indicated that this sequence is composed of tandemly arranged imperfect repeats of the consensus 5 AATGG 3 similar to previously identified satellite III sequences. Digestion of D15Z1 with HinfI resulted in a series of restriction fragments making up a subset of the HinfI ladder components of satellites III and IV. These data suggest that D15Z1 represents a chromosome 15 specific domain of human satellites III or IV and that it makes up the major fraction of the heterochromatin of this chromosome. Possible relationships between this sequence and the cytochemical staining properties of human chromosomes with distamycin A/DAPI, D280/170, and antiserum to 5-methylcytosine are discussed.     

Glia development in the embryonic CNS of Drosophila.

Glial cells are pivotal players during the development and function of complex nervous systems. In Drosophila, recent genetic analyses have revealed several genes that control differentiation and function of CNS glial cells and their interactions with neurons can be studied in detail at the CNS midline, where it is essential for the correct establishment of the commissural axon pattern.     

Commissure formation in the embryonic CNS of Drosophila.

In the ventral nerve cord of Drosophila most axons are organized in a simple, ladder-like pattern. Two segmental commissures connect the hemisegments along the mediolateral and two longitudinal connectives connect individual neuromeres along the anterior-posterior axis. Cells located at the midline of the developing CNS first guide commissural growth cones toward and across the midline. In later stages, midline glial cells are required to separate anterior and posterior commissures into distinct axon bundles. To unravel the genes underlying the formation of axon pattern in the embryonic ventral nerve cord, we conducted a saturating ethylmethane sulfonate mutagenesis, screening for mutations which disrupt this process. Subsequent genetic and phenotypic analyses support a sequential model of axon pattern formation in the embryonic ventral nerve cord. Specification of midline cell lineages is brought about by the action of segment polarity genes. Five genes are necessary for the establishment of the commissures. In addition to commissureless, the netrin genes, and the netrin receptor encoded by the frazzled gene, two gene functions are required for the initial formation of commissural tracts. Over 20 genes appear to be required for correct development of the midline glial cells which are necessary for the formation of distinct segmental commissures.     

A 40 kb chromosomal fragment encoding Salmonella typhimurium invasion genes is absent from the corresponding region of the Escherichia coli K-12 chromosome

Many Salmonella typhimurium genes are required for bacterial entry into host cells. P22 transduction analysis has localized several invasion loci near minute 59 on the S. typhimurium chromosome. To further characterize the 59–60 min chromosomal region, we determined the physical and genetic map of 85 kb of S. typhimurium DNA between srl and cysC. It was previously shown that some of the invasion genes from this region are not present in Escherichia coli K-12. We examined whether other S. typhimurium genes on the 85 kb of DNA were similarly absent from E. coli We found that a contiguous 40 kb fragment of the S. typhimurium chromosome which encodes invasion genes is absent from the corresponding region of the E. coli K-12 chromosome and may represent a pathogenicity island. We speculate that acquisition of the 40 kb region must have significantly advanced the evolution of Salmonella as a pathogen.     

不同致病型马立克氏病病毒1.8kb基因家族序列比较

为了分析鸡马立克氏病病毒(Marek's disease virus, MDV)的致病性与其1.8kb基因家族的关系,本研究比较了四个不同致病型12个毒株的该基因家族的同源性关系,即弱毒疫苗株、强毒株、超强毒株、特超强毒株和中国野毒株.结果表明不同毒株间1.8kb基因家族的上游调控序列的同源性在1.8kb基因家族转录方向上大于92.5%,序列间有缺失突变,其中大多数突变发生在弱毒株上.CVI988在TATA box 和上游SP1位点间丢失小片段"5′CTCGG 3′".1.8kb基因家族中存在132bp串连重复序列,CVI988包含3-7个132bp串连重复序列拷贝,强毒株、超强毒株、特超强毒株通常只包含2个串连重复序列.但是已知的中国疫苗毒株814株也只有2个重复序列,表明重复序列的拷贝数不是引起病毒毒力变化的主要原因.各毒株的1.8kb基因家族同源性在97%以上.其中未剪切的1.69kb cDNA中有两个阅读框,分别编码63和64个氨基酸组成的多肽.不同毒株的这二个多肽都很保守,虽然也有一些氨基酸变异,但这些变异与病毒的致病型或地域分布没有明显的关系.     

Ozone inhalation activates stress-responsive regions of the CNS

Ozone (O(3)), a major component of air pollution, has considerable impact on public health. Besides the well-described respiratory tract inflammation and dysfunctions, there is accumulating evidence indicating that O(3) exposure affects brain functions. However, the mechanisms through which O(3) exerts toxic effects on the brain remain poorly understood. This work aimed at precisely characterizing CNS neuronal activation after O(3) inhalation using Fos staining in adult rat. We showed that, together with lung inflammation, O(3) exposure caused a sustained time- and dose-dependent neuronal activation in the dorsolateral regions of the nucleus tractus solitarius overlapping terminal fields of lung afferents running in vagus nerves. Furthermore, we highlighted neuronal activation in interconnected central structures such as the caudal ventrolateral medulla, the parabrachial nucleus, the central nucleus of the amygdala, the bed nucleus of the stria terminalis and the paraventricular hypothalamic nucleus. In contrast, we did not detect any neuronal activation in the thoracic spinal cord where lung afferents running in spinal nerves terminate. Overall, our results demonstrate that O(3) challenge evokes a lung inflammation that induces the activation of nucleus tractus solitarius neurons through the vagus nerves and promotes neuronal activation in stress-responsive regions of the CNS.     

Glia dictate pioneer axon trajectories in the Drosophila embryonic CNS

Whereas considerable progress has been made in understanding the molecular mechanisms of axon guidance across the midline, it is still unclear how the axonal trajectories of longitudinal pioneer neurons, which never cross the midline, are established. Here we show that longitudinal glia of the embryonic Drosophila CNS direct formation of pioneer axon pathways. By ablation and analysis of glial cells missing mutants, we demonstrate that glia are required for two kinds of processes. Firstly, glia are required for growth cone guidance, although this requirement is not absolute. We show that the route of extending growth cones is rich in neuronal cell bodies and glia, and also in long processes from both these cell types. Interactions between neurons, glia and their long processes orient extending growth cones. Secondly, glia direct the fasciculation and defasciculation of axons, which pattern the pioneer pathways. Together these events are essential for the selective fasciculation of follower axons along the longitudinal pathways.     

Inscuteable-independent apicobasally oriented asymmetric divisions in the Drosophila embryonic CNS

Inscuteable is the founding member of a protein complex localised to the apical cortex of Drosophila neural progenitors that controls their asymmetric division. Aspects of asymmetric divisions of all identified apicobasally oriented neural progenitors characterised to date, in both the central and peripheral nervous systems, require inscuteable. Here we examine the generality of this requirement. We show that many identified neuroblast lineages, in fact, do not require inscuteable for normal morphological development. To elucidate the requirements for apicobasal asymmetric divisions in a context where inscuteable is not essential, we focused on the MP2 > dMP2 + vMP2 division. We show that for MP2 divisions, asymmetric localisation and segregation of Numb and the specification of distinct dMP2 and vMP2 identities require bazooka but not inscuteable. We conclude that inscuteable is not required for all apicobasally oriented asymmetric divisions and that, in some cellular contexts, bazooka can mediate apicobasal asymmetric divisions without inscuteable.