The Effect of Cell Population Density on the Amino Acid Requirements for Poliovirus Synthesis in Hela Cells

I t has previously been shown that in full grown monolayer cultures of HeLa cells infected with poliovirus the only components of the medium necessary for maximum virus output are glucose, glutamine, and salts (1, 2). Rappaport (3) has similarly reported maximum poliovirus production in monkey kidney cultures in the absence of a number of growth-essential amino acids; and a number of growth-essential amino acids were found by Bader and Morgan (4) to be non-essential in the production of psittacosis virus by the L cell. The question remains as to whether, in a medium lacking nutritionally essential amino acids, poliovirus protein is synthesized from the pre-existing free amino acids of the pool, or whether instead there is a breakdown of cell protein to products which are then used for viral synthesis. An approach to this problem has been provided by a study of the effect of cell population density on the nutritional requirements for poliovirus synthesis by HeLa cells. In two other virus cell systems it has already been shown that dilution of the infected cell suspension in saline greatly reduces the virus production per cell. Chicken fibroblasts infected with Western equine encephalomyelitis and suspended in Earle's saline at a low population density require embryo extract in order to produce virus normally (5). Pereira (6) has shown that when chick fibroblasts infected with fowl plaque virus are suspended in saline the virus yield per cell is higher at high population densities. These findings suggest that cellular components essential for virus synthesis may be lost when cells are suspended at low concentrations in a deficient medium.