The RNA-dependent DNA polymerase present in intracisternal A-type particles from mouse myeloma tumor cells has been studied. This polymerase can use either endogenous A particle RNA or an exogenous synthetic polynucleotide [poly(rA) ] as a template. The DNA reaction product is small (4S-1OS) and over 90% of it hybridizes to A particle RNA, whereas up to 50% of it hybridizes to murine sarcoma-leukemia virus RNAs. The RNA isolated from purified A particles is generally of low molecular weight (5S-15S) but contains small amounts of 70S and 35S components. These results suggest that A-type particles may be related to C-type oncornaviruses.

Oncornavirus-like particles of the "A" (both intracisternal and intracytoplasmic) and "B" or "C" (extracellular) types are produced by murine MOPC-MATERIALS AND METHODS Reagents and viruses. Triton X-100, sucrose (grade I, RNase free), actinomycin D, RNase A, DNase I, deoxynucleoside triphosphates (dATP, dCTP, dGTP, dTTP), dithioerythritol, and bovine 344

The structure of the purified replicative intermediate of encephalomyocarditis virus was determined by electron microscopy. Approximately 80% of the replicative intermediate complexes were characterized by a filament of double-stranded RNA of widely variable length, which had a "bush" of single-stranded RNA at one end. In many examples one or more additional singlestranded bushes were appended internally to the doublestranded RNA filament. These results support the view that before deproteinization, replicative intermediate contains little if any double-stranded RNA.Double-stranded RNA is formed in vivo during the replication of picorna and other RNA-containing viruses. This material has been implicated in the viral inhibition of host-cell protein biosynthesis, and it may play a role in the induction of interferon. While early work suggested that double-stranded RNA formed an integral part of the RNA replication complex (termed "replicative intermediate," RI) this idea has more recently been questioned (1-3). The possibility has been raised that double-stranded RNA may be formed only as a byproduct of replication (e.g., "replicative form," RF), and its presence in RI has been termed an artifact that occurs during purification. A similar view has been expressed regarding the role of double-stranded RNA in the replication of RNA bacteriophages (1,2). This theory has recently been confirmed for R17 phage by the demonstration that the replicating complexes, in particular those containing RI, are composed almost entirely of single-stranded RNA (4). Double-stranded RNA was formed in these structures only as a result of heating or deproteinization. The electron microscopic techniques used in that study have now been applied to the replication of encephalomyocarditis (EMC) virus, and the results are described in the present communication. As in the case of R17 RNA replication, the issue can be stated in terms of two conflicting models for the structure of RI. The older model (model A, Fig. 1), involving double-stranded RNA, and the newer singlestranded model (model B) predict slightly different structures for purified (deproteinized) RI. In model A, deproteinized RI (RI-A') contains a long "backbone" of double-stranded RNA of constant length with two free ends, and one or more "bushes" of single-stranded RNA appended internally; in the newer model (model B), however, deproteinized RI (RI-B') contains a shorter double-stranded region of variable length with only one free end, a bush at the other end, and any number of bushes in between. These two structures, RI-A' and RI-B', are readily distinguishable in the electron microscope by recently developed techniques (4). The results of the present analysis show that the structure of EMC RI is identical to the hypothetical RI-B'. From this it may be concluded that RI contains little or no double-stranded RNA before deproteinization. This result implies that the only extensively double-stranded viral RNA in infected cells must be in the form of RF. METHQDS EMC virus an...

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