Citrobacter freundU encodes an inducible chromosomal P-lactamase similar to the constitutively expressed ampC .8-lactamase of Escherichia coli. In the latter species the ampC gene is located next to the fumarate reductase (frd) operon, whereas in C. freundii the ampC gene is known to be separated from frd by 1100 base pairs. P3-lactamase (8, 9). This enzyme acts by either hydrolyzing or by simply binding the ,B-lactam that reaches the periplasmic space (10-12).The ampC gene of Citrobacter freundii has been cloned and was shown to be closely linked to the frd operon, which codes for the fumarate reductase complex (4), as is the case in Escherichia coli (13). In C. freundii, however, there is an 1100-base-pair (bp) DNA segment, not present in E. coli, that separates ampC from frd (4). We show that this segment codes for a trans-acting 31-kilodalton protein, AmpR, which is responsible for the inducibility of,-lactamase synthesis from the C. freundii ampC gene, and that it can have both a positive and a negative effect on ampC expression. We further show that mutations leading to high constitutive levels of ampC f3-lactamase production map outside the ampR-ampC region. MATERIALS AND METHODSMedia and Growth Conditions. L medium (14) was used for routine purposes. M9 medium (15) containing 0.2% glucose, 0.2% casamino acids, uracil at 50 ,ug/ml, and thiamin at 1 ,ug/ml (M9CA) was used in the determination of ampicillin resistance and .-lactamase expression. When necessary, the media were supplemented with ampicillin (concentration as indicated), tetracycline (10 ,ug/ml), or chloramphenicol (10 Ag/ml).Determination of Ampicillin Resistance and Relative ,-Lactamase Production. Ampicillin resistance on M9CA medium was determined as the LD50-i.e., the concentration of ampicillin required to inhibit 50% of the single cells from forming colonies (16). To determine the relative production of ,B-lactamase, cells were washed twice (50 mM potassium phosphate buffer, pH 7.4) and sonicated (17). (3-Lactamase activity was determined spectrophotometrically (18) at 255 nm, using 100 uM cephalosporin C as substrate in 10 mM MgCl2/10 mM potassium phosphate buffer, pH 7.4. Specific activity was expressed as ,mol of substrate hydrolyzed per min at 22°C per mg of total protein (19).Induction of f-Lactamase Expression in C. freundu and E. coli. Bacteria were grown exponentially in M9CA medium for at least eight generations to an OD420 of 0.8. Subsequently, they were diluted into an equal volume of prewarmed medium containing twice the final inducer concentration. Samples, 20 or 40 ml, were withdrawn at various times into centrifuge tubes on ice, containing chloramphenicol (200 ,g/ml) to inhibit protein synthesis. Relative 83-lactamase production was then determined.Induction of ,B-Lactamase Expression in Minicells. Strain AA10, obtained from P. Orndorff (Stanford University), is a recA derivative of P678-54 (20) and was used for the minicell experiments. Minicells from AA10 harboring the approprate plasmids were prepared (21) a...

14N spin-lattice (7\) and spin-spin (T2) relaxation times have been measured in aqueous solutions of nhexadecyltrimethylammonium bromide (CTAB) and chloride (CTAC) as functions of the concentration. It is confirmed that CTAC forms spherical micelles at all concentrations above the cmc while CTAB at higher concentration forms larger aggregates. The thermal motions of the surfactant molecules can be described by a local rapid anisotropic motion and a slower motion over distances of the order of the dimension of the aggregates. The fast motion is characterized by a correlation time rcf ~5.6 X 1010 s in CTAB micelles and it is approximately twice as fast in CTAC micelles. The correlation time for the slow motion in CTAB micelles increases rapidly with concentration. From comparison with models for the motion of rodlike aggregates it is concluded that the large micelles probably are flexible.Publication costs assisted by the National Research Council of Canada Ab initio calculations with a STO-3G and a 6-31G basis set have been performed on vinyl cyanide and isocyanide to establish optimized geometries and the associated electronic structures and energies. The energy of conversion of the cyanide to the isocyanide is calculated as 17.66 kcal mol"1. The ionic character of the isocyanide appears to be a factor of 2 larger than that for the cyanide. Calculations with a two-parameter transition model for the conversion of cyanide to isocyanide produced an approximately isosceles triangular configuration at the barrier peak corresponding to 87.5 and 69.9 kcal mol"1 for conversion from the cyanide and isocyanide, respectively.The ionic character of the transition state is much smaller than that of the cyanide. No evidence for the existence of a metastable transition state was obtained.

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Kraft lignin gels have been found to exhibit both macrosyneresis and hysteresis in swelling. The effects of temperature and prehistory on swelling and on the mechanical properties have been investigated. Thermal treatment of kraft lignin gels in the protonised state induces an irreversible deswelling of the gels. This irreversible deswelling can, however, be released by deprotonization of the carboxyi groups. Deswelling also occurs when partly dried protonised gels are placed in water again. Furthermore, the gels were found to exhibit pH-hysteresis. It is concluded that the above-mentioned effects are closely related to the state of dissociation of the carboxylic groups and to their ability to form intermolecular hydrogen bonds in the network structure. It is suggested that syneresis is due to a structural rearrangement induced by breaking and formation of hydrogen bonds and promoted by the long-range van der Waal's interaction between the colloidal units in the gel. Swelling hysteresis is assumed to be related to repeptization phenomena commonly encountered in lyophobic colloids.

SynopsisThe swelling kinetics of cellulose hydrogels have been studied in experiments where partially dried gels were reswollen to equilibrium in liquid water. The swelling interval studied was from the dry state to 3.6 g water/g dry gel, and the temperature range was from 15 to 65OC. The experimental arrangement and the theoretical analysis was based on the unidimensional, unsteady sorption of water into a swelling sheet. The effects of sheet thickness, temperature and the initial degree of swelling were investigated. The integral sorption curves obtained were sigmoid in shape when plotted as the sorbed amount of water versus the square root of time. These anomalous, non-Fickian sorption curves were analyzed under the assumption that the anomalous behavior is due to a slow establishment of concentration equilibrium at the boundary surfaces. Each sorption curve can then be described by a combination of a mutual diffusion coefficient and a surface relaxation rate. The sigmoidicity of the sorption curves is accordingly determined by the ratio of a characteristic diffusion time to a characteristic surface relaxation time. More classical, Fickian-type behavior is then explained by an increase in this ratio. The experimental sorption curves were found to become more nearly classical as the sheet thickness, the temperature and/or the initial degree of swelling was increased. The relaxation process was found to be associated with a higher apparent activation energy than the diffusion process.