High resolution scanning electron microscopy at the subcellular level

Recently developed scanning electron microscopes provide sufficient resolution to allow useful observation of subcellular biological objects. Preparation methods for such objects need not be limited to the traditional coating and mounting procedures. Many methods developed for transmission electron microscopy are immediately adaptable to scanning electron microscopy. We show that a number of techniques are available to the microscopist which yield adequate contrast and high resolution. As examples we show skeletal muscle myofibrils dispersed to reveal thick filaments, uncoated on a thin carbon film; a tropomyosin tactoid, negatively stained with uranyl acetate; oncornavirus, conventionally coated; and T4 bacteriophage on an aluminium substrate.

Recombinant DNA molecules of bacteriophage phi chi174

Phi chi174 DNA structures containing two different parental genomes were detected genetically and examined by electron microscopy. These structures consisted of two monomeric double-stranded DNA molecules linked in a figure 8 configuration. Such DNA structures were observed to be formed preferentially in host recA+ cells or recA+ cell-free systems. Since the host recA+ allele is required for most phi chi174 recombinant formation, we conclude that the observed figure 8 molecules are intermediates in, or end products of, 1 phi chi174 recombination event. We propose that recombinant figure 8 DNA molecules arise as a result of "single-strand aggression," are stabilized by double-strand "branch migration," and represent a specific example of a common intermediate in genetic recombination.

In situ lysis of phi29- and SPO1-infected Bacillus subtilis

An improved method of in situ lysis of bacteriophage-infected Bacillus subtilis was developed and used to study phi29 and SPO1 phage structures produced by individual cells.

phi gamma: A coliphage coliphage related to, but distinct from the phi 80 virion

The coliphage phi gamma, though capable of genetic recombination with phi 80, is morphologically distinct from the phi 80 virion. It has a prolate head (58.4 X 46.7 nm) bearing a tail (143 nm) which is strikingly flexible. On the basis of their buoyant density in CsC1, both infectious and transducing phi gamma particles form a single population. This density value is slightly higher than that of the phi 80 virion. The phi gamma chromosome is a double-stranded linear DNA molecule of 13.4 mum in length (corresponding mol. wt.: 27.6 X 10(6)). From its melting temperature and its buoyant density in CsC1, phi gamma DNA appears to have a base composition very close to that of Escherichia coli DNA.

Assembly of the tail of bacteriophage T4

The protein products of at least 21 phage genes are needed for the formation of the tail of bacteriophage T4. Cells infected with amber mutants defective in these genes are blocked in the assembly process. By characterizing the intermediate structures and unassembled proteins accumulating in mutant-infected cells, we have been able to delineate most of the gene-controlled steps in tail assembly. Both the organized structures and unassembled proteins serve as precursors for in vitro tail assembly. We review here studies on the initiation, polymerization, and termination of the tail tube and contractile sheath and the genetic control of these processes. These studies make clear the importance of the baseplate; if baseplate formation is blocked (by mutation) the tube and sheath subunits remain essentially unaggregated, in the form of soluble subunits. Seventeen of the 21 tail genes specify proteins involved in baseplate assembly. The genes map contiguously in two separate clusters, one of nine genes and the other of eight genes. Recent studies show that the hexagonal baseplate is the end-product of two independent subassembly pathways. The proteins of the first gene cluster interact to form a structure which probably represents one-sixth of the outer radius. The products of the other gene cluster interact to form the central part of the baseplate. Most of the phage tail precursor proteins appear to be synthesized in a nonaggregating form; they are converted to a reactive form upon incorporation into preformed substrate complexes, without proteolytic cleavage. Thus reactive sited are limited to growing structures.