Molecular weight of DNA from coliphage T7 by electron microscopy

Comparative studies on the structural proteins of T3 and T7 phages

A comparison of the coat protein patterns of the wild types of the related phages T3 and T7 on SDS polyacrylamide gel electrophoresis was carried out. After infection of the nonpermissive host with T7 amber mutants in genes 7, 11, 12, 13 or 17 and T3 amber mutants in genes 11, 12, 13 and 17 respectively, noninfectious, DNA containing particles were produced. The protein pattern, as well as electron microscopy of defective particles of T3 and T7 led to the conclusion that the proteins specified by genes 11, 12, 13 and 17 are tail proteins whereas the proteins coded by genes 7, 8, 10, 14, 15 and 16 enter the head structure. The "serum blocking protein" (gene 17 product) seems to play a different role in the assembly of T3 and T7 tails, since T3 particles defective in gene 17 did not show any tail structure connected with the head whereas T7 particles defective in gene 17 had a tail which looked different from that of the wildtype. Treatment of wildtype particles with ammonium nitrate or sodium pyrophosphate led to morphologically abberrant forms which had partially or completely lost the hexagonal head structure. After treatment with ammonium nitrate ball-like structures were obtained, both for T3 and T7. However, in the case of T3 these abberrant forms contained the proteins specified by genes 7, 8 and 10 whereas those derived from T7 contained only the proteins specified by genes 8 and 10. Sodium pyrophosphate treatment of T3 and T7 wildtype particles led to a release of the phage tails. The isolated tails were examined by electron microscopy thus revealing for the first time a detailed substructure of the T3 and T7 phage tails. In order to find out more about the tail proteins, absorption experiments with isolated bacterial cell walls were carried out.

Practical procedures for the purification of bacterial viruses

The efficiencies of the various methods used for phage concentration have been compared. The two-phase concentration method (with polyethylene glycol and dextran sulfate) gave maximal recoveries of infectivity for coliphages of the T-even and T-odd series and for ribonucleic acid phages and single-stranded deoxyribonucleic acid phages. Precipitation of phages by acid gave high yields when applied to T2 and T4 phages but not with T3 and T7 coliphages. Differential centrifugation was efficient when sedimented phages were gently dispersed before repeating the centrifugation cycle. The efficiencies of the various methods have also been confirmed by electron microscope studies, which also show that the two-phase concentration method gave rise to intact phages. Zone centrifugations in sucrose gradients (12.5 to 52.5%) indicated that coliphages of the T-even series sediment faster than T-odd coliphages; they may thus be separated from each other and from empty ghosts by centrifugation at 100,000 x g for 40 min. Equilibrium centrifugation in preformed cesium chloride gradients was also useful for phage concentration and purification. This study also deals with some optical properties of purified phages; optical cross sections and absorbance ratios (at 260 and 280 nm) of the various preparations are given.

Studies of nondefective adenovirus 2-simian virus 40 hybrid viruses. 3. Base composition, molecular weight, and conformation of the Ad2+ND genome

The nondefective adenovirus 2 (Ad2)-simian virus 40 (SV40) hybrid virus, Ad2(+)ND(1), differs from the defective Ad-SV40 hybrid populations previously described, in that this hybrid virus can replicate without the aid of nonhybrid adenovirus helper. Consequently, the deoxyribonucleic acid (DNA) from this virus, which can be obtained free of nonhybrid adenovirus DNA, is well suited for biophysical studies on Ad-SV40 hybrid DNA. Such studies have been performed and demonstrate Ad2(+)ND(1) DNA to have a buoyant density (1.715 g/cm(3)) and thermal denaturation profile (T(m) = 75.1 C) almost identical with nonhybrid Ad2 DNA. Furthermore, its molecular weight, as determined by analytical zone sedimentation and electron microscopy, was 22 x 10(6) to 25 x 10(6) daltons, which is also very similar to that determined for Ad2. Electron micrographs showed all of the hybrid molecules to be double-stranded and linear. By using this determination of the molecular weight of Ad2(+)ND(1) DNA and assuming that 1% of this molecule consists of covalently linked SV40 DNA (see companion paper), we calculate that the hybrid DNA molecule contains 220 x 10(3) to 250 x 10(3) daltons of SV40 DNA, or the equivalent of one-tenth of the SV40 genome.