The Nature of Iron in Ferredoxin.

to detect hybrid particles. Although the RNA of dependent virus often replicated to a level about 10 per cent of that of supporting virus,' maturation of intact dependent virus within heterologous capsid was much less efficient. This could be due to a better fit between capsomeres and homologous RNA or it could be due to a spatial relationship between capsomere protein synthesis and the viral RNA which serves both as the template for such synthesis and as a nucleus for capsomere aggregation into capsid. A definite sequence of replication of viral RNA and protein may well be required for efficient encapsidation of RNA. This would explain why heterologous viral RNA superinfecting cells late after infection by another virus is not enclosed within the capsid protein being synthesized by the first virus (Table 1). Compartmentalization of virus within the cell' might also explain this failure. Finally, it is obvious that hybrid virus of this type offers a simple tool to determine whether virus host range and tissue tropism is determined by virus capsid affinity for cell receptors, as has been suggested earlier.6 Summary.-Under certain in vivo conditions of double infection, the protein capsid of a replicating type 2 poliovirus or B. Coxsackie virus enclosed the RNA of type 1 poliovirus to form millions of mature "hybrid" virus particles whose immunologic phenotype differed from their genotype.