Five to seven minutes after addition of
14
C-amino acids to poliovirus-infected MiO cells, radioactivity is found only in 5
S
virus-specific structures. With increased time of labeling, radioactivity appears in considerable amounts also in the 14
S
area. In the presence of guanidine blocking the transition of 14
S
particles in subsequent structures in MiO cells, radioactivity is first detected in the 5
S
zone and then accumulates predominantly in 14
S
particles. These results indicate that formation of 5
S
particles and conversion of a part of their protein into 14
S
particles reflect the earliest stages of the morphogenesis of poliovirus. In contrast to poliovirus-infected HeLa cells, no 73
S
particles could be detected in MiO cells, nor could 73
S
structures be detected in poliovirus-infected MiO cells after a short incubation period (20 to 30 min) with radioactive amino acids, although formation of 150
S
virions was observed. Addition of guanidine to infected MiO cells leads to accumulation of 14
S
particles; however, 73
S
particles are not detected. After removal of guanidine, radioactivity increases only in the 150
S
area. Incubation of 14
S
particles isolated from the sucrose gradient of the cytoplasmic extract of infected MiO cells resulted in formation of 73
S
particles. The results obtained show the modification of the late stages of poliovirus morphogenesis in MiO cells.
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