Abstract
L cells (mouse fibroblasts), uninfected and infected with Chlamydia psittaci (meningopneumonitis strain), were labeled with (14)C-amino acids, and their membranous organelles were separated by isopycnic equilibrium centrifugation of whole cell homogenates on discontinuous sucrose density gradients. Incorporation of labeled amino acids into host and parasitic proteins was differentiated on the basis of susceptibility to cycloheximide. Twenty hours after infection with C. psittaci, incorporation of newly synthesized proteins into the internal membranes of L cells was almost completely inhibited, and internal membranes made prior to infection were altered or destroyed. The unit membrane that at all times surrounds the cytoplasmic vacuole containing the multiplying chlamydiae was made by the host from membranes or membrane precursors present before infection. No proteins synthesized by C. psittaci became associated with host cell membranes. Destruction or modification of the internal membranes of the host cell may be an integral part of the chlamydial developmental cycle.