(1) The RNA replicase induced by bacteriophage Qbeta consists of four 
non-identical subunits designated as alpha (mol. wt. 74000), beta (mol. wt. 
64000), gamma (mol. wt. 47000) and delta (mol. wt. 33000), only one (subunit 
beta) of which is specified by the phage genome. (2) Subunit alpha (30 S 
ribosomal protein "S1" as well as translational interference factor "i") is 
required only for (+) strand-directed RNA synthesis in the presence of the host 
factor. (3) Qbeta replicase lacking subunit alpha (R-alpha) is capable of 
replicating templates other than (+) strand, such as (--), "6S" RNA, poly(C) 
etc., in the absence of the host factor. (4) Subunit beta is suggested to be the 
nucleotide-polymerizing enzyme, but is unable to initiate RNA synthesis by 
itself. (5) Subunits gamma and delta are identical to the protein synthesis 
elongation factors, EF-Tu and EF-Ts, respectively, and are required only for 
initiation of RNA synthesis, but not for elongation. (6) A model of Qbeta 
replicase is presented in order to discuss observed template-enzyme 
interactions.