Tryptic fragments of the Escherichia coli DNA gyrase A protein

Treatment of the Escherichia coli DNA gyrase A protein with trypsin generates two large fragments which are stable to further digestion. The molecular masses of these fragments are 64 and 33 kDa, and they are shown to be derived from the N terminus and the C terminus of the A protein, respectively. These fragments could represent structural and/or functional domains within the A subunit of DNA gyrase. The trypsin-cleaved A protein (A'), in combination with the B subunit of gyrase, can support ATP-dependent supercoiling of relaxed DNA and other reactions of DNA gyrase. The isolated 64-kDa fragment will also catalyse DNA supercoiling in the presence of the B protein, but the 33-kDa fragment shows no enzymic activities. We conclude that the N-terminal 64-kDa fragment represents the DNA breakage/reunion domain of the A protein, while the 33-kDa fragment may contribute to the stability of the gyrase-DNA complex.

DNA gyrase can supercoil DNA circles as small as 174 base pairs

DNA gyrase introduces negative supercoils into closed-circular DNA using the free energy of ATP hydrolysis. Consideration of steric and thermodynamic aspects of the supercoiling reaction indicates that there should be a lower limit to the size of DNA circle which can be supercoiled by gyrase. We have investigated the supercoiling reaction of circles from 116-427 base pairs (bp) in size and have determined that gyrase can supercoil certain relaxed isomers of circles as small as 174 bp, dependent on the final superhelix density of the supercoiled product. Furthermore, this limiting superhelical density (-0.11) is the same as that determined for the supercoiling of plasmid pBR322. We also find that although circles in the range 116-152 bp cannot be supercoiled, they can nevertheless be relaxed by gyrase when positively supercoiled. These data suggest that the conformational changes associated with the supercoiling reaction can be carried out by gyrase in a circle as small as 116 bp. We discuss these results with respect to the thermodynamics of DNA supercoiling and steric aspects of the gyrase mechanism.

Short-term group therapy with depressed adolescent outpatients

A selected review of the literature about short-term therapy and social skills as they may apply to depressed adolescents is presented. Two forms of group therapy are described. These are social skills and a traditional discussion group format. Some of the difficulties in establishing and evaluating a group therapy program are outlined.

Will Nfld.'s masterplan eradicate the shortage of nurses?

As many Canadian provinces and most parts of the United States are reacting to a serious shortage of nurses, the province of Newfoundland and Labrador has chosen a proactive - rather than reactive - approach to dealing with this critical issue.

DNA supercoiling by DNA gyrase. A static head analysis

Using purified DNA gyrase to supercoil circular plasmid pBR322 DNA, we examined how the linking number attained at the steady state ('static head') varies with the concentrations of ATP and ADP, both in the absence and presence of spermidine. In the absence of spermidine at total adenine nucleotide concentrations between 0.35 and 1.4 mM, the static-head linking number was independent of the sum concentration of ATP and ADP, but depended strongly on the ratio of their concentrations. We established that the same linking number was attained independent of the direction from which the steady state was approached. The decrease in linking number at static head is more extensive when spermidine is present in the incubation, but remains a function of the [ATP]-to-[ADP] ratio. These results are discussed in terms of various kinetic schemes for DNA gyrase. We present one kinetic scheme that accounts for the experimental observations. According to this scheme our experimental results imply that there is significant slip in DNA gyrase when spermidine is absent. It is possible that spermidine acts through adjustment of the degree coupling of DNA gyrase.

Structure of the DNA gyrase-DNA complex as revealed by transient electric dichroism

We have analyzed the structure of complexes between DNA gyrase and four defined DNA fragments by electric dichroism. Both the extrapolated dichroism and relaxation time of these complexes suggest that a single turn of DNA is wrapped around the enzyme with the entry and exit points located close together. The average angle between the DNA tails emerging from the particle is about 120 degrees. This structure is consistent with that seen by electron microscopy. Addition of ATP or the non-hydrolyzable ATP analog 5'-adenylyl-beta, gamma-imidodiphosphate results in a structural change of the complex, consistent with the DNA tails now being wrapped around the protein. The significance of these observations with respect to the mechanism of DNA supercoiling by DNA gyrase is discussed.

B protein of bacteriophage mu is an ATPase that preferentially stimulates intermolecular DNA strand transfer

A DNA strand-transfer reaction is an early step in the transposition of phage Mu. It has been shown that an efficient reaction in vitro requires, in addition to buffer and salt, only the Mu A protein, Mu B protein, host protein HU, ATP, and Mg2+. We have determined that, of the three protein factors involved, only the Mu B protein has an ATPase activity. The Mu B ATPase is stimulated by Mu A protein and DNA but not by either of these factors alone. Double-stranded DNA is a much better cofactor than single-stranded DNA, but there is no apparent sequence specificity. In the absence of the Mu B protein and/or ATP, the intermolecular Mu DNA strand-transfer reaction is extremely inefficient, and the strand-transfer products are predominantly the result of an intramolecular reaction. This contrasts with the efficient intermolecular reaction that occurs if Mu B protein and ATP are provided. The Mu B protein, in the presence of Mu A protein and protein HU, therefore, seems to facilitate interactions between potential DNA target sites and pairs of Mu DNA ends.

Co-operativity and enzymatic activity in polymer-activated enzymes. A one-dimensional piggy-back binding model and its application to the DNA-dependent ATPase of DNA gyrase

The binding of a ligand to a one-dimensional lattice in the presence of a second ("rider") ligand, which binds only to the first ligand (piggy-back binding), is studied. A model derived from this study is used to analyze the effects of co-operativity on the reaction rates of enzymes activated by polymeric cofactors that provide multiple binding sites for the enzyme. It is found that in the presence of strong co-operativity, the steady-state reaction rates of polymer-activated enzymes can be very different from the Michaelis-Menten paradigm. By adjusting the co-operativity parameters and the binding constants of the ligands, the model can generate apparent auto-catalytic enhancement by substrates at low substrate concentrations and apparent substrate inhibition at high substrate concentrations. The model is shown to be able to explain the differences in the rates of ATP hydrolysis by DNA gyrase in the presence of long versus short DNA molecules and in the presence of long DNA molecules at different gyrase to DNA ratios.

The DNA dependence of the ATPase activity of DNA gyrase

We have studied the ATPase activity of DNA gyrase both in the absence and presence of DNA. In the absence of DNA we show that the gyrase B protein alone has a very low level of ATPase activity which can be increased many-fold by pretreatment of the B protein with heat or urea. When both the gyrase A protein and linear DNA are also present, the ATPase activity of the untreated B protein is greatly stimulated. We find that the extent of stimulation is dependent upon the length of the DNA but largely independent of DNA sequence. DNA molecules greater than 100 base pairs in length are much more effective in stimulating the gyrase ATPase than those of 70 base pairs or less, although short DNA molecules will stimulate the ATPase at high concentrations. The behavior of long and short DNA molecules with respect to ATPase stimulation is also reflected in their abilities to bind DNA gyrase. To account for these data we propose a model for the interaction of gyrase with ATP and DNA in which ATP hydrolysis requires the binding of DNA to two sites on the enzyme.

The SalGI restriction endonuclease. Mechanism of DNA cleavage

The cleavage of supercoiled DNA of plasmid pMB9 by restriction endonuclease SalGI has been studied. Under the optimal conditions for this reaction, the only product is the linear form of the DNA, in which both strands of the duplex have been cleaved at the SalGI recognition site. DNA molecules cleaved in one strand at this site were found to be poor substrates for the SalGI enzyme. Thus, both strands of the DNA appear to be cleaved in a concerted reaction. However, under other conditions, the enzyme cleaves either one or both strands of the DNA; the supercoiled substrate is then converted to either open-circle or linear forms, the two being produced simultaneously rather than consecutively. We propose a mechanism for the SalGI restriction endonuclease which accounts for the reactions of this enzyme under both optimal and other conditions. These reactions were unaffected by the tertiary structure of the DNA.

The SalGI restriction endonuclease. Enzyme specificity

We have analysed the kinetics of DNA cleavage in the reaction between the SalGI restriction endonuclease and plasmid pMB9. This reaction is subject to competitive inhibition by DNA sequences outside the SalGI recognition site; we have determined the Km and Vmax. for the reaction of this enzyme at its recognition site and the KI for its interaction at other DNA sequences. We conclude that the specificity of DNA cleavage by the enzyme is only partly determined by the discrimination it shows for binding at its recognition sequence compared with binding to other DNA sequences.

The SalGI restriction endonuclease. Purification and properties

The type II restriction endonuclease SalGI has been purified to near homogeneity. At least 80% of the protein remaining after the final stage of the preparation is SalGI restriction endonuclease; no contaminating nucleases remain detectable. The principal form of the protein under both native and denaturing conditions is a monomer of M(r) about 29000. The optimal conditions for both enzyme stability and enzyme activity have been determined.

Corynebacterium endophthalmitis. Laboratory studies and report of a case treated by vitrectomy

A Corynebacterium sp was isolated from the vitreous humor under aseptic conditions on two separate occasions from a patient with endophthalmitis resulting from a penetrating injury by a metallic foreign body. The metallic foreign body was removed during a pars plana lensectomy and anterior vitrectomy procedure; intravitreously injected methicillin sodium, gentamicin sulfate, and dexamethasone sodium phosphate gave a functional visual result. Intravitreous inoculation of rabbits with the Corynebacterium isolate produced an endophthalmitis similar to that produced in the patient, and subsequent cultures from the vitreous of the inoculated rabbits grew the same Corynebacterium sp. To our knowledge, this is the first reported case of endophthalmitis in which a Corynebacterium sp was documented by intraocular culture.

Figure and ground, methodology, and the spiral aftereffect

The color of ground against which a black spiral pattern was seen to rotate was a significant factor in determining duration of latency but not duration of aftereffect. Reasons are given why the tachistoscopic method of stimulus presentation employed by the authors is considered superior to other methods of stimulus presentation used in spiral aftereffect research.