Identification of an essential second metal ion in the reaction mechanism of Escherichia coli adenylosuccinate synthetase

This study reports that two Mg2+ ions are required for Escherichia coli adenylosuccinate synthetase activity. The first metal ion is presumably coordinated with beta- and gamma-phosphoryl groups of GTP to provide an electron sink, and the second one seems to interact with aspartate in the enzyme active site. Regarding the latter metal ion, kinetic studies show that aspartate and the second Mg2+ ion bind to the enzyme active site randomly with a kcat value of 1.47 s-1 and with Km values for aspartate and Mg2+ of 225 and 114 microM, respectively. The dissociation constants for aspartate and Mg2+ of the enzyme.GTP.IMP.(aspartate or Mg2+) complex are 79.2 and 40.0 microM, respectively. However, variable amounts of aspartate or Mg2+ did not show any significant changes in the Km values for GTP and IMP. Kinetic studies using Mn2+ and Ca2+ ions indicate that the kcat values (0.930 and 0.235 s-1, respectively) were slightly decreased compared with the value obtained using Mg2+; however, the Km values for aspartate and GTP in the presence of Mn2+ and Ca2+ were significantly decreased compared with those obtained using Mg2+ ion (4.5 and 4.6 times for Mn2+ ion and 5.6 and 5.8 times for Ca2+ ion, respectively). On the other hand, the Km values for IMP were not significantly changed (1.9 and 1.8 times for Mn2+ and Ca2+ ions, respectively). Taken together, these kinetic results imply that aspartate may interact with Mg2+ to form a Mg.aspartate complex in the enzyme active site. An inhibition study of the enzyme with ZnCl2 (its Ki value is 29 nM) also suggested that Zn2+ competes with aspartate as well as Mg2+, implying that Zn2+ might form a complex with aspartate in the active site. On the basis of these results, it is suggested that Mg.aspartate complex formation in the active site of adenylosuccinate synthetase may be important in activation of the protonated amino group of aspartate, enhancement of the enzyme's binding affinity, and its specificity for aspartate.

Crystal structure of the T4 regA translational regulator protein at 1.9 A resolution

The translational regulator protein regA is encoded by the T4 bacteriophage and binds to a region of messenger RNA (mRNA) that includes the initiator codon. RegA is unusual in that it represses the translation of about 35 early T4 mRNAs but does not affect nearly 200 other mRNAs. The crystal structure of regA was determined at 1.9 A resolution; the protein was shown to have an alpha-helical core and two regions with antiparallel beta sheets. One of these beta sheets has four antiparallel strands and has some sequence homology to RNP-1 and RNP-2, which are believed to be RNA-binding motifs and are found in a number of known RNA-binding proteins. Structurally guided mutants may help to uncover the basis for this variety of RNA interaction.

[Surgical treatment of penoscrotal transposition associated hypospadias]

Thirty-four patients with penoscrotal transposition associated with hypospadias were surgically treated from 1978 to 1992. Penoscrotal hypospadias was noted in 23, and perineal hypospadias in 11. Incomplete penoscrotal transposition was scen in 24 patients, and complete penoscrotal transposition in 10. 32 patients out of 34 obtained satisfactory results after the correction of penoscrotal transposition. Complication occurred in 5 patients (urethral fisula 3 and orifice structure 2). The operative procedures of penoscrotal transposition were discussed.

Extension of the four-stranded intercalated cytosine motif by adenine.adenine base pairing in the crystal structure of d(CCCAAT)

The crystal structure of d(CCCAAT), refined at 2.0 A resolution, shows a four stranded molecule in which two parallel duplexes intercalate with opposite polarity, using cytosine.protonated cytosine base pairs. The intercalation motif in this structure is extended by adenine.adenine base pairs. Two topologically distinct broad grooves are found in the lath-like central part of the molecule with the phosphate groups on one side bent over towards each other, stabilized by bridging water molecules. At the 3' ends, two arrangements of intermolecular A.A.T base triplets are found, involving both asymmetric and symmetric A.A base pairs joined to thymine residues by Watson-Crick and reverse Hoogsteen base pairing, respectively.

Stable loop in the crystal structure of the intercalated four-stranded cytosine-rich metazoan telomere

In most metazoans, the telomeric cytosine-rich strand repeating sequence is d(TAACCC). The crystal structure of this sequence was solved to 1.9-A resolution. Four strands associate via the cytosine-containing parts to form a four-stranded intercalated structure held together by C.C+ hydrogen bonds. The base-paired strands are parallel to each other, and the two duplexes are intercalated into each other in opposite orientations. One TAA end forms a highly stabilized loop with the 5' thymine Hoogsteen-base-paired to the third adenine. The 5' end of this loop is in close proximity to the 3' end of one of the other intercalated cytosine strands. Instead of being entirely in a DNA duplex, this structure suggests the possibility of an alternative conformation for the cytosine-rich telomere strands.

Identification and characterization of a human cDNA homologous to yeast SKI2

A monoclonal antibody, 170A1, which recognizes a nucleolar peptide of molecular weight 90,000, was raised. The protein was conserved among various vertebrates. To characterize the antigen, we screened a human fetal liver expression library using the monoclonal antibody as a probe. Molecular analyses of immunopositive clones suggested the presence of a novel cDNA. It appeared to be a single-copy gene and encoded about 4- and 5-kb mRNAs. The gene appeared to be expressed in every cell tested so far. Its deduced amino acid sequence revealed an overall homology to recently described yeast SKI2. The SKI2 gene of Saccharomyces cerevisiae encoded a nucleolar protein that is involved in the antiviral system. We report here the partial human cDNA sequence and the localization of the corresponding gene on chromosome 6p21.

Antisense RNA-mediated inhibition of mouse hepatitis virus replication in L2 cells

We have successfully used antisense RNA to inhibit replication of the mouse hepatitis virus (MHV) in a cell culture system. MHV is a single-stranded RNA virus of positive polarity. Mouse L2 cells were stably transfected with an antisense construct that targets regions of genes 5 and 6 of the virus. High levels of expression from this construct, which is under control of the human elongation factor 1 alpha promoter, were found. After infection of the antisense cell lines with MHV, replication of the virus was significantly reduced compared with control cells. In a viral plaque assay, smaller plaques were found in the antisense cell lines. In addition, up to a 92% inhibition in the number of viral particles produced in one antisense cell line could be seen. This inhibitory effect decreased at longer (> 16 hour) infection times. It was possible to both increase the amount of inhibition and prolong the inhibitory effect by reducing the multiplicity of infection. Our results suggest that antisense RNA may be an effective tool to slow down progression of MHV infection in mice.

Start site selection at lacUV5 promoter affected by the sequence context around the initiation sites

The effects of single base pair substitutions at the initiation sites of lacUV5 promoter on the transcription start site selection by E. coli RNA polymerase were systematically studied. Transcription start sites were mapped by sizing the cytosine-specifically terminated transcripts produced in vitro by using a chain terminator 3'-deoxycytidine 5'-triphosphate (3'-dCTP) in transcription reactions. Transcription of a prototype lacUV5 promoter initiated with three purines (-1G, +1A and +2A; +1 representing the predominant start site) located 6-8 bp downstream from the Pribnow box. All the substitutions affected the start site selection, resulting in a change in the number of start sites (from 3 to 2 or 1) and/or a shift of the major start site (to -1 or +2). None of the variants started outside the 3-bp region and at the positions substituted by a pyrimidine. Purine-to-pyrimidine changes suppressed not only initiation at the substituted position but also, in some cases, at the other purine position. Purine-to-purine changes also shifted the major start site or suppressed the initiation at other sites. Changes at -2 and +5 also affected the start site selection. Thus, the sequence context around the initiation sites of lacUV5 promoter strongly influences the selection of initiating nucleotides by E. coli RNA polymerase.

Replacement of Asp333 with Asn by site-directed mutagenesis changes the substrate specificity of Escherichia coli adenylosuccinate synthetase from guanosine 5'-triphosphate to xanthosine 5'-triphosphate

The aspartate residue of the (N/T)KXD concensus sequence for GTP-binding proteins is present in the eight available sequences of adenylosuccinate synthetase. Reported here is a comprehensive analysis of the substrate specificity of mutant enzymes, where the conserved Asp333 of the synthetase from Escherichia coli is changed to asparagine, glutamate, and glutamine by site-directed mutagenesis. The mutants D333N, D333E, and D333Q generally show decreased kcat values and increased Km values for GTP. The decreased values of kcat exhibited by the mutants indicate that the interactions between Asp333 and the guanine are relayed by some mechanism to the catalytic residues around the gamma-phosphate of GTP, and that the energy provided by the interaction between Asp333 and the guanine moiety of GTP is utilized for rearrangement of the catalytic residues. The three mutants each have higher affinity for xanthosine 5'-triphosphate (XTP) and ITP than does the wild-type enzyme. In fact, the D333N mutant uses XTP more effectively than the wild-type enzyme employs GTP as a substrate. The side-chain of Asp333 forms hydrogen bonds with the N-1 and the exocyclic amino group of the guanine base of GTP. In the D333N mutant, this interaction is probably replaced by hydrogen bonds between the amide side chain of Asn333 and N-1 and the 2-oxo group of XTP. The D333Q mutant can use UTP as a substrate more effectively than the wild-type enzyme. The longer side chain of glutamine at residue 333 favors pyrimidine nucleotides over the purine nucleotides, GTP, XTP, and ITP. These results demonstrate that Asp333 in the (N/T)KXD consensus sequence of adenylosuccinate synthetase from E. coli is a determinant for GTP-specificity.

Cloning and sequence analysis of the gene encoding the low potential cytochrome c of Synechocystis PCC 6803

The gene for the low potential cytochrome c (petK) was isolated from a genomic library of Synechocystis PCC 6803. The nucleotide sequence of this gene contains two regions with sequence similarity to two regions in the gene for the high potential cytochrome c6 of the organism. The sixth iron ligand can be identified with a conserved histidine. Experiments demonstrate the reduction of the low potential cytochrome by reduced ferredoxin II. The heme of the cytochrome is flanked by lysines which may be involved in orienting the ferredoxin near the site of electron donation.

Characterization of the putative GTP-binding site residues of Escherichia coli adenylosuccinate synthetase by site-directed mutagenesis

Adenylosuccinate synthetase contains amino acid sequences in its GTP-binding domain that are homologous to other G-proteins. This homology includes a glycine-rich phosphate-binding loop, GXXXXGK, and a guanine-specific binding region, (N/T/Q)KXD; however, virtually no other sequence homology exists between other G-proteins and adenylosuccinate synthetase. On the basis of X-ray diffraction studies, the folding topologies of the synthetase and the p21 ras proteins are different. Yet, residues that interact with GTP in the p21 ras proteins are present in the synthetase in nearly identical positions. We chose therefore to study the G15V mutant, a phosphate-binding loop mutant, and K331L and K331R, two mutants of Lys331 that are involved in guanine ring binding. The Km values for GTP of adenylosuccinate synthetase mutants, K331L and K331R, when compared to those of the wild-type enzyme, were 27- and 20-fold increased, respectively, without any significant change in the Km values for IMP. Because both mutations affected the Km values for GTP similarly, whereas the kcat and secondary structure were essentially unchanged, it is suggested that Lys331 is located in the GTP-binding site of adenylosuccinate synthetase and the terminal N zeta of the Lys is not necessarily important in GTP-binding on the enzyme. Therefore, Lys331 may interact with GTP through hydrophobic interactions between its linear side chain and the aromatic ring of the guanine base of GTP. Also, structural characterization of the G15V mutant was carried out using circular dichroism (CD) spectrometry, NMR spectroscopy, and spectrofluorometry. The CD spectral data indicated that the secondary structure of the G15V mutant was significantly altered by GTP and IMP, whereas that of the wild-type enzyme is not changed; however, the two enzymes exhibited similar secondary structures in the absence of substrates. The NMR spectra of both enzymes were also similar in the absence of substrates. The dissociation constant (Kd) for IMP of the G15V mutant was 4.8-fold larger than its Km value which was 1.5-fold increased compared to the wild-type enzyme. From these findings, it was concluded that the phosphate-binding region of adenylosuccinate synthetase is involved in a conformational change induced by GTP and IMP binding, and that GTP and IMP binding depend on the presence of the other substrate at the active site of the enzyme. These results suggest that the Lys331 of adenylosuccinate synthetase may play similar roles in the function and structure to that of GTP-binding proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of chromosome aberrations in leiomyoma and leiomyosarcoma using FISH on archival tissues

Fluorescence in situ hybridization (FISH) with chromosome-specific probes was used to study cytogenetic changes in five cases of leiomyosarcoma (LMS) and nine cases of uterine leiomyoma (LM). Biotinylated DNA probes for the centromeric regions of chromosomes 1, 6, 8, 9, 17, and 18, painting probes for chromosomes 1 and 22, and the cosmid probe for chromosome region 21q22.3 were used on nuclei isolated from paraffin blocks. Four of five LMS cases revealed major chromosomal aberrations, while the only case with minor clonal aberrations was subsequently found not to be a typical LMS. The most common numerical aberrations found in the LMS cases were extra copies of chromosome 8 (three of five cases), loss of chromosome 1 (three of five cases), and loss of chromosome 6 (two of five cases). One of two LMS cases studied with a chromosome 1 painting probe demonstrated translocations of chromosome 1. In contrast to LMS, only five of nine uterine LM cases had abnormal clones, and these were smaller than those in LMS. Two LM cases showed 9% tetrasomy 8 with 17 or 20% monosomy 6, and three other cases had monosomy 6 clones in 18-34% of cells. These results indicate that typical LMS is characterized by multiple chromosomal aberrations affecting most of the cells, whereas borderline LMS and LM have fewer affected chromosomes and less clonal involvement.

All 4 bases of both strands at -9 and -8 in T7 promoter are needed to be substituted by SP6-specific bases to switch promoter specificity

It has previously been shown that a substitution of two base pairs at -9 and -8 in T7 promoter by SP6 promoter-specific base pairs alone is sufficient to make the T7 variant become an SP6 promoter, abolishing T7 promoter activity [Lee, S. S., and Kang, C. (1992) Biochem. Int. 26, 1-5]. In order to determine which of the 4 bases on both strands at the two positions is involved in the discriminative specificity, each base or each pair of bases was substituted by SP6-specific base(s), allowing one or two mismatches. The results show that all-four-base substitution only is capable of switching promoter specificity, and that all the four bases are substantially involved in T7 promoter activity.

One-step insertion of oligonucleotide linkers or adapters to DNA using unphosphorylated oligonucleotides

A simple and efficient method was developed for insertion of oligonucleotide sequences into plasmids. In this method, an unphosphorylated oligonucleotide was ligated to the restriction-digested phagemid DNA. Only the single strand of the oligonucleotide was ligated at the 5' end of the phagemid, and this resulted in the creation of a long self-complementary single-strand overhang. These single-strand overhang-possessing phagemids were used to transform XL-1 cells. This simple ligation and transformation reaction rendered approximately 7.5 x 10(4) to 5 x 10(5) of white colonies per microgram DNA from the isopropyl-beta-D-thiogalactopyranoside and 5-bromo-4-chloro-3-indolyl-beta-D-galacto-pyranoside plate. This number is almost the same or even higher than the number of blue colonies from the control reaction in which ligase was used without the oligonucleotide. By this method we could mutate one enzyme site to another or create ribozyme and substrate phagemid very easily. Fidelity of this method was checked by restriction digestion, DNA sequencing and ribozyme reaction. By DNA sequencing, we observed that 100% of the white colonies contained a single oligonucleotide sequence.

Two base pairs at -9 and -8 distinguish between the bacteriophage T7 and SP6 promoters

Bacteriophage T7 and SP6 RNA polymerases and their promoters share a high degree of their primary structure homology, but each polymerase exclusively recognizes its own promoter sequence. To reveal the molecular basis of this specificity, 4 base pairs at positions -12, -10, -9, and -8 of the T7 promoter were substituted individually and multiply by SP6 promoter-specific base pairs, and 3 base pairs at -10, -9, and -8 of the SP6 promoter were replaced by T7 promoter-specific base pairs. Promoter activities of 28 sequences were measured in vitro with T7 and SP6 polymerases separately under optimal conditions at 6 mM MgCl2. Single and double substitutions at -12 and -10 do not significantly affect the T7 promoter activity, although they are almost exclusively conserved among T7 genomic promoters. Changes at -10 of SP6 promoter hardly affect the activity. However, any T7 variants that contain either or both changes at -9 and -8 show greatly reduced activity. Interestingly, the double substitution at -9 and -8 yields significant SP6 promoter activities and virtually no T7 promoter activity. Furthermore, the SP6 promoter variants with both T7-specific -9C and -8T show good T7 promoter activities, although they still show some SP6 promoter activities. However, under high salt conditions (either 20 mM MgCl2 or 100 mM NaCl plus 6 mM MgCl2), they show only slight SP6 promoter activity. No other SP6 variants show any T7 promoter activity. All these results indicate that the 2 base pairs at -9 and -8 of both the T7 and SP6 promoters are the primary (if not the only) determinants of specificity and that the hierarchy of importance of positions for promoter activity is -8, -9 > > -10 > -12. Also, a phylogenic relationship among the T3, T7, K11, and SP6 promoters is suggested based on dissimilarities in their sequences from -12 to -8.

Effects of multiple mutations at the conserved TATA sequence of bacteriophage SP6 promoter on transcription efficiency

Mutations of A<==>T were introduced individually and multiple to TATA from -4 to -1 of the phage SP6 promoter and their effects on transcription initiation efficiency measured in vitro. All 15 mutants tested were less active than the wild type. Mutation at -4T nearly abolishes promoter activity independent of other changes, and alteration at -3A reduces promoter activity substantially. On the other hand, effects of mutations at -2T and -1A depend on other changes, suggesting their role should be associated with neighboring base pairs. These results suggest that -4T and -3A are involved in SP6 RNA polymerase binding and -2T and -1A are involved in DNA unwinding. This bipartite role of the SP6 promoter TATA contrasts with the single role of T7 promoter TATA on DNA unwinding. The polymerase binding region extends further downstream in the SP6 promoter than in the T7 promoter.

Iron(II)/hydroperoxide (Fenton reagent)-induced activation of dioxygen for (A) the direct ketonization of methylenic carbon and (B) the dioxygenation of cis-stilbene

Several iron complexes [FeII(PA)2 (PA = picolinate), FeII(bpy)2(2+), FeII(OPPh3)4(2+), FeII(MeCN)4(2+), (Cl8TPP)FeII(py)2 (Cl8TPP = tetrakis(2,6-dichlorophenyl)porphyrin), and FeIIICl3] in combination with R'OOH (R' = H, t-Bu) catalytically activate O2 to oxygenate hydrocarbons [e.g., c-C6H12-->c-C6H10(O) [9 O2 turnovers per FeII(PA)2 or FeII(bpy)2(2+), and 13 per (Cl8TPP)-FeII(py)2]; PhCH2CH3-->PhC(O)CH3 (up to 25 O2 turnovers per FeIILx); c-C6H10-->c-C6H8(O) (up to 9 O2 turnovers per FeIILx); PhCH(Me)2-->PhC(O)Me, Ph(Me)2COH, and Ph(Me)C = CH2 (up to 5 O2 turnovers per FeIILx); and cis-PhCH = CHPh-->2PhCH(O) (up to 2 O2 turnovers per FeLx)]. With large R'OOH/FeLx ratios spontaneous decomposition occurs to give free O2 that is incorporated into the substrates. The product profiles for the various FeIILx/R'OOH, O2/RH systems and their electrochemical characterization during steady-state turnover confirm that the first-formed intermediate is a one-to-one R'OOH/FeIILx adduct (e.g., [(PA)2-FeIIOOR' + pyH+] (1)) (Fenton reagent), which reacts with (a) excess FeII(PA)2 to give (PA)2FeIIIOR', (b) excess c-C6H12 to give (c-C6H11)py (kinetic isotope effect, [KIE] = kc-C6H12/kc-C6D12, 4.6 with t-BuOOH and 1.7 with HOOH), (c) excess R'OOH to give [(PA)2FeIV(OH)(OOR')] (3), then [(PA)2FeIV(O2)] (7) and O2, and (d) O2 to form an adduct, [(PA)2-FeIII(O2)(OOR') + pyH+] (5), that reacts with c-C6H12 to form c-C6H10(O), [K] = 8.2 (t-BuOOH) and 2.1 (HOOH). When PhCH2CH3 or c-C6H10 are the substrates (RH), 5 reacts to form [(PA)2FeIV(OH)(OOR)] (6), which in turn reacts with RH and O2 in a catalytic cycle to give PhC(O)Me or c-C6H8(O) [up to 7 O2 turnovers per iron with FeII(OPPh3)4(2+)]. Species 7 reacts with cis-PhCH = CHPh to give PhCH(O).

A comparison of techniques in the assessment of chest wall thickness and composition

The thickness and composition of defined regions of the anterior chest wall are important factors in the assessment of pulmonary plutonium by low-energy x ray counting. Estimates of these quantities are reported for seven male subjects investigated by three laboratories using ultrasonic methods and by a fourth laboratory using magnetic resonance imaging. No important bias was found in any one laboratory's estimates of chest wall thickness relative to those of the others, but differences of up to 6 mm were noted for individual subjects. The discrepancies are believed principally to reflect the different sampling regimes adopted to reach a representative mean chest wall thickness over the region of interest from measurements at selected points. The adipose-tissue component was consistently found to be lower when assessed by magnetic resonance imaging compared with estimates by ultrasound, but the differences were unimportant in the context of plutonium assessment.

Crystal structure of four-stranded Oxytricha telomeric DNA

The sequence d(GGGGTTTTGGGG) from the 3' overhang of the Oxytricha telomere has been crystallized and its three-dimensional structure solved to 2.5 A resolution. The oligonucleotide forms hairpins, two of which join to make a four-stranded helical structure with the loops containing four thymine residues at either end. The guanine residues are held together by cyclic hydrogen bonding and an ion is located in the centre. The four guanine residues in each segment have a glycosyl conformation that alternates between anti and syn. There are two four-stranded molecules in the asymmetric unit showing that the structure has some intrinsic flexibility.

Transcript RNA having trans-acting antitermination activity on the T7 transcription terminator

The efficiency of the phage T7 intrinsic terminator was determined in pulse-labeling in vitro transcription reactions. While the factor-independent terminator subcloned in pET3a showed consistently high (approximately 80%) efficiency, the efficiency of the same terminator in pGEM3ZT was initially approximately 60% but exponentially decreased to approximately 20%, although the 39-bp terminator, and its 73-bp upstream and 32-bp downstream sequences are identical in the two plasmids. When transcription product mixture of pGEM3ZT was added to an on-going reaction of pET3a, the terminator efficiency from pET3a was immediately reduced to approximately 40%. Furthermore, when the pGEM3ZT product mixture was subjected to the promoter-cleaving HinfI digestion and then phenol/chloroform extraction, the mixture still maintained the trans-acting antitermination activity. The results suggest that the trans-acting component(s) are RNA synthesized from pGEM3ZT.

A two-base-pair substitution in T7 promoter by SP6 promoter-specific base pairs alone abolishes T7 promoter activity but reveals SP6 promoter activity

The phage T7 and SP6 RNA polymerase-promoter systems are very similar in many characteristics, but maintains stringent specificity for each. In order to identify the base pair element that distinguishes between T7 and SP6 promoters, the base pairs at -12, -10, -9, and -8 of the T7 promoter consensus sequence were changed singly and multiply to the SP6 promoter-specific base pairs, and assayed for T7 and SP6 promoter activities. The results indicate that the primary discrimination element is the base pairs at -8 and -9. The two-base-pair substitution alone in T7 promoter by SP6-specific base pairs is sufficient to make the T7 variant be a SP6 promoter, abolishing T7 promoter activity.

Activity of chloramphenicol acetyltransferase overproduced in E. coli with wild-type and mutant GroEL

The homo-oligomeric protein chloramphenicol acetyltransferase (CAT) has previously been shown to interact with a chaperone GroEL in vitro, suggesting a possible involvement of GroEL in CAT assembly. CAT was overproduced to various levels in the presence and absence of GroEL overproduction, and in groEL mutants. CAT was accumulated to 9-45% of total cellular protein in a fully soluble form, without formation of inclusion bodies. In all cases, even with groEL mutants, CAT specific activity was shown proportional to the amount of protein produced, indicating the formation of active trimer CAT structure does not need GroEL in Escherichia coli.

[Physico-chemical analysis of Ostreas gigas Thumberg from Xinghua]

This paper presents a comparative physico-chemical analysis of the ancient Ostrea gigas with the medical Ostrea gigas concha, and shows that the protein and amino acid contents in the former are obviously lower than those in the latter, but for the contents of trace elements Mn, Fe, Ni and Pb (with the exception of As), the former appears higher than the latter.

A mesh/token ring hybrid-architecture LAN

This paper presents a hybrid architecture for local area networks (LANs) which combines the advantages of token ring and mesh networks while remaining relatively simple. Each node may have two communication channels. The first is a token ring and is used for short or prioritized message data transmissions. The other consists of an arbitrary collection of data links, which are used for relatively high volume and low priority data messages. With this architecture, a LAN can be installed in the form of token ring and can be upgraded and “tuned” to its application environment by the insertion of node-to-node data links.

Simulation results indicate that even with rather simple network control and routing algorithms, performance is improved over token ring due to data transmission concurrency greater than one.

Studies on SP6 promoter using a new plasmid vector that allows gene insertion at the transcription initiation site

All the phage-promoter containing subcloning vectors available for in vitro transcription reactions contain a polylinker away from the transcription initiation site. A new SP6 transcription subcloning vector, pCKSP6, has been constructed, in which a gene can be inserted precisely at the transcription initiation site. This was achieved by bringing the BamHI cleavage site into the initiation site. When DNA ends of both insert gene and BamHI cleaved pCKSP6 are made blunt-ended using a single strand specific nuclease, the in vitro transcripts of the recombinant DNA by SP6 RNA polymerase will contain only the gene sequence immediately after the initiation base G. Mung bean nuclease was used to generate a series of mutants resulting from step-wise deletion of single base pairs around the initiation site. Transcription assays with these SP6 promoter mutants revealed that not only the sequence immediately upstream of the initiation site but also the six base pairs from position +1 to +6 are important elements for promoter binding and/or transcription initiation activity. Furthermore, there appears to be a hierarchy of importance of each base pair in the order of position +1 greater than +2 greater than +3 greater than +4, +5, +6, -1, -2.

Enzymatic synthesis of uniformly 32P-labeled polyribonucleotides and high-specific-activity ribonucleoside 5'-[alpha-32P]diphosphates

Uniformly 32P-labeled polyribonucleotides of high specific activity can be rapidly and easily synthesized from commercially available ribonucleoside 5'-[alpha-32P]triphosphates by using two enzymes in sequence. Myosin ATPase completely and irreversibly converted any triphosphates to diphosphates in 10 min. The product diphosphates, without purification, can be polymerized by polynucleotide phosphorylase (PNPase) in 1 h with an average yield of 60%. By choosing the desired molar ratio of radioactive and nonradioactive tri- or diphosphates, polymers of a wide range of specific activity can be obtained. Since myosin ATPase and PNPase both have little base specificity, the method can be used to synthesize a radiolabeled polymer of any desired base composition.

A fluorescent derivative of ribosomal protein S18 which permits direct observation of messenger RNA binding

70 S Escherichia coli ribosomes were reacted with the fluorescent dye N-(iodoacetylaminoethyl)-5-naphthylamine-1-sulfonic acid for 10 min under mild conditions. The resulting ribosomes were fully active. 30 S subunits isolated from these particles were also fully active. They contain approximately 0.7 eq of fluorescent dye. Nearly all of it is attached to protein S18. Competitive reaction with N-ethylmaleimide implies that the fluorescent dye is located at cysteine 10 of the protein. The labeled 30 S particles will recombine with 50 S subunits to form stable 70 S particles. Thus the procedures we have developed allow the large scale preparation of an active fluorescent conjugate of the 70 S ribosome. The fluorescence of the 70 S particles is sensitive to the binding of mRNA, showing both quenching and a shift in emission spectra. Thus it affords a simple way to quantitate mRNA binding directly. In pilot studies without tRNA, the binding constant of the initiation triplet codon adenylyl-(3' leads to 5')-uridylyl-(3' leads to 5')-guanosine to 70 S ribosome was found to be an order of magnitude larger than that of polyuridylic acid.