Different recombination site specificity of two developmentally regulated genome rearrangements

In the absence of a combined nitrogen source, such as ammonia, approximately every tenth vegetative cell along filaments of the cyanobacterium Anabaena develops into a heterocyst, a terminally differentiated cell that is morphologically and biochemically specialized for nitrogen fixation. At least two specific DNA rearrangements involving the nitrogen-fixation (nif) genes occur during heterocyst differentiation, one within the nifD gene and the other near the nifS gene. The two rearrangements have several properties in common. Both occur quantitatively in all heterocyst genomes, both occur at approximately the same developmental time, late in the process of heterocyst differentiation, and both result from site-specific recombination between short repeated DNA sequences. We report here the nucleotide sequences found at the site of recombination near the nifS gene. These sequences differ from those found previously for the nifD rearrangement, suggesting that the two rearrangements are catalysed by different enzymes and may be regulated independently. We also show that the nifS gene is transcribed only from rearranged genomes.     

Organization and evolution of the class I gene family in the major histocompatibility complex of the C57BL/10 mouse

The major histocompatibility complex (MHC) encodes several classes of protein vital to the regulation of the immune response. We have isolated 26 class I genes that map to this region in the C57BL/10 mouse and linked these into three gene clusters. The number of genes differs from the number found in the BALB/c strain and comparison of the organization of the class I genes in these two strains shows conserved regions and polymorphic regions which probably result from deletions, insertions and translocations within the MHC.     

Rearrangement of nitrogen fixation genes during heterocyst differentiation in the cyanobacterium Anabaena

Nitrogen fixation by the cyanobacterium Anabaena is carried out in heterocysts, specialized, non-dividing cells which differentiate under conditions of ammonia or nitrate deprivation. In Anabaena, heterocyst differentiation is accompanied by rearrangement of some nitrogen fixation genes. A site-specific recombination between an 11 base-pair direct repeat sequence flanking the nifK and nifD genes removes 11 kilobases of intervening DNA, resulting in juxtaposition of the two genes and an alteration of the nifD protein-coding sequence.     

psbA genes indicate common ancestry of prochlorophytes and chloroplasts

It has long been suspected that chloroplasts evolved after an endosymbiotic event involving a photosynthetic prokaryote, presumably a cyanobacterium, and a eukaryotic organism. Recent studies have provided strong evidence about the cyanobacterial nature of chloroplasts. Since the discovery of prochlorophytes, oxygen-evolving photosynthetic prokaryotes containing chlorophyll a and chlorophyll b and lacking phycobiliproteins, there has been speculation that these represent evolutionary intermediates between cyanobacteria and chloroplasts. Prochloron sp., the first described prochlorophyte, proved difficult to work with because it is an obligate symbiont of marine ascidians. Prochlorothrix hollandica, a recently isolated, freshwater filamentous prochlorophyte, is easily maintained in the laboratory. Overall pigment composition and thylakoid membrane structure of P. hollandica suggest it has intermediate characteristics between cyanobacteria and the chloroplasts of higher plants. The P. hollandica psbA genes, which encode the photosystem II thylakoid protein D1, were cloned and sequenced and the sequences compared to those reported for cyanobacteria, a green alga, a liverwort, and several higher plants. The two psbA genes present in P. hollandica encode an identical amino-acid sequence. As in all chloroplast psbA genes, there is a seven amino-acid gap near the C terminus of the derived protein relative to the protein predicted by cyanobacterial genes, suggesting that P. hollandica is part of the lineage that led to chloroplasts after a divergence from cyanobacteria. This hypothesis is also supported by phylogenetic analysis of derived D1 amino-acid sequences from psbA genes of thirteen taxa on the basis of parsimony.     

Expression of murine H-2Kb histocompatibility antigen in cells transformed with cloned H-2 genes

Cosmids containing H-2 histocompatibility antigen genes of the H-2b haplotype have been isolated. One of these genes expresses a 45,000 molecular weight protein, indistinguishable from H-2Kb when introduced into mouse L cells. These H-2Kb transformed L cells can be killed by allospecific anti-H-2Kb cytotoxic T cells. Moreover, when infected with influenza virus, they can be killed by an H-2Kb-restricted, influenza virus-specific cytotoxic T cell line. These results show that expression of the H-2Kb gene product on the L-cell surface is sufficient to make it a target for specific T-cell killing.     

氧化钒簿膜电极在锂充电电池中的应用(英文)

鉴于氧化钒是应用于锂充电电池的最有希望的电极材料之一，采用等离子体增强化学气相沉积法（ＰＥＣＶＤ）和激光溅射沉积法（ＰＬＤ），制备了不同的氧化钒薄膜，并研究了它们的电化学性质。这些薄膜具备较高的充放电容量和优良的充放电稳定性。晶体和非晶体的Ｖ２Ｏ５薄膜都可以用ＰＬＤ方法在２００℃和不同的气氛下制备。这些薄膜的充电容量可达３８０ｍＡ·ｈ／ｇ。所制备的最稳定的薄膜是用ＰＥＣＶＤ方法得到的。这些薄膜的Ｏ／Ｖ比很接近Ｖ６Ｏ１３（厚度约为０．５ｕｍ）。它们的放电容量可达４０８ｍＡ．ｈ／ｇ或１２６５ｍＡｈ／ｃｍ３，其能量密度可达９６０．３Ｗ．ｈ／ｋｇ。即使经过４４００次的充放电后，这些薄膜的放电容量仍基本不变。因此，ＰＥＣＶＤ方法及其用它所制备的氧化钒薄膜将是锂充电电池工业很有希望的一种选择。     

Structure of a tyrosyl-tRNA synthetase splicing factor bound to a group I intron RNA

The 'RNA world' hypothesis holds that during evolution the structural and enzymatic functions initially served by RNA were assumed by proteins, leading to the latter's domination of biological catalysis. This progression can still be seen in modern biology, where ribozymes, such as the ribosome and RNase P, have evolved into protein-dependent RNA catalysts ('RNPzymes'). Similarly, group I introns use RNA-catalysed splicing reactions, but many function as RNPzymes bound to proteins that stabilize their catalytically active RNA structure. One such protein, the Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (TyrRS; CYT-18), is bifunctional and both aminoacylates mitochondrial tRNA(Tyr) and promotes the splicing of mitochondrial group I introns. Here we determine a 4.5-A co-crystal structure of the Twort orf142-I2 group I intron ribozyme bound to splicing-active, carboxy-terminally truncated CYT-18. The structure shows that the group I intron binds across the two subunits of the homodimeric protein with a newly evolved RNA-binding surface distinct from that which binds tRNA(Tyr). This RNA binding surface provides an extended scaffold for the phosphodiester backbone of the conserved catalytic core of the intron RNA, allowing the protein to promote the splicing of a wide variety of group I introns. The group I intron-binding surface includes three small insertions and additional structural adaptations relative to non-splicing bacterial TyrRSs, indicating a multistep adaptation for splicing function. The co-crystal structure provides insight into how CYT-18 promotes group I intron splicing, how it evolved to have this function, and how proteins could have incrementally replaced RNA structures during the transition from an RNA world to an RNP world.