ABSTRACT:A new method for exact analytical calculation of the accessible surface areas and their gradients with respect to atomic coordinates is described. The new surface routine, GETAREA, finds solvent-exposed vertices of intersecting atoms, and thereby avoids calculating buried vertices which are not needed to determine the accessible surface area by the Gauss᎐Bonnet theorem. The surface routine was implemented in FANTOM, a program for energy minimization and Monte Carlo simulation, and tested for accuracy and efficiency in extensive energy minimizations of Met-enkephalin, the ␣-amylase inhibitor Ž . tendamistat, and avian pancreatic polypeptide APP . The CPU time for the exact calculation of the accessible surface areas and their gradients has been Ž . Ž . reduced by factors of 2.2 Met-enkephalin and 3.2 tendamistat compared with our previous approach. The efficiency of our exact method is similar to the recently described approximate methods MSEED and SASAD. The performance of several atomic solvation parameter sets was tested in searches for low energy conformations of APP among conformations near the native X-ray crystal structure and highly distorted structures. The protein solvation parameters from w Ž .

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Although the virulence of Bacterium tularense may be maintained for prolonged periods of cultivation on artificial media, a considerable reduction in v-irulence has been frequently observed. As early as 1922 Francis reported the loss of vTirulence of certain strains of B. tularense following serial transfer on seium glucose agar. Foshay (1932) described the loss of virulence of two strains isolated from fatal human cases following prolonged cultivation on coagulated egg yolk medium. Ransmeier (1943) reported a reduction of virulence after serial transfer of an originally highly virulent strain on glucose cystine blood agar. It is also well known that repeated passage of cultures through a given animal species may result in a marked increase in virulence for the particular host (Philip and Davis, 1935). Thus, changes in virulence have been amply demonstrated; however, it has not been possible to correlate these changes with any other easily detectable characteristics. All attempts to correlate cellular morphology with virulence (Hesselbrock and Foshay, 1945; Eigelsbach, Chambers, and Coriell, 1946) have met with failure. Because various bacterial species display a high degree of correlation between colonial morphology, pathogenicity, and immunogenic properties (Braun, 1947), an attempt was made to determine whether such correlations may also exist for B. tularense. As far as the authors are aware, studies on the colonial morphology of B. tularense have not been reported previously in the literature. MATERIALS AND METHODS The majority of experiments were initiated with either a highly virulent culture, Schu, originally isolated by Foshay in 1941 from a human ulcer, or with an avirulent culture, 38, which was originally isolated by Francis in 1920 from a human lymph node and subsequently became totally avirulent (Hesselbrock and Foshay, 1945). These cultures, as well as variant types subsequently isolated, were maintained on glucose cysteine blood agar (GCBA) at 5 to 10 C following a 24-hour growth period at 37 C. The solid transparent medium used for viable count and colony type observations consisted of 2 per cent Difco peptone, 1 per cent NaCl, 0.1 per cent glucose, 0.1 per cent cysteine hydrochloride, and 2 per cent agar with a final reaction of pH 6.8 (Snyder et al., 1946). The basal liquid medium used was identical in composition except for the absence of agar. Vliable counts were made by plating serial dilutions on GCBA; total growth was determined by packed cell volume in modified hematocrit tubes after centrifugation at 4,000 rpm for 1 hour. Colony types wvere inspected under a dissecting microscope with the help of obliquely transmitted light, achieved by placing the mirror, concave side up, 557

Complexes of polyadenylic and polyuridylic acids, or of polycytidylic acid and methylated bovine serum albumin, enhance the early rate of increase in numbers of antibody-forming spleen cells in mice immunized with sheep red blood cells or other particulate antigens. Polyadenylic and polycytidylic acids, respectively, appear to be the source of the critical stimulators which, as demonstrated by others in bacteria, may act by influencing nucleotide kinase activity. The stimulated antibody response, but not the normal response, is antagonized by kinetin riboside and by an adenosine derivative occurring in sRNA.