(2′‐5′)A n ‐dependent endoribonuclease: Enzyme levels are regulated by IFNβ, IFNγ, and cell culture conditions

The levels of a (2′‐5′) A n ‐dependent endonuclease (RNase L) were determined in extracts prepared from murine L cells and Ehrlich ascites tumor (EAT) cells by measuring specific binding of protein to a labeled derivative of (2′‐5′)A n , (2′‐5′)A 3 [ 32 P]pCp. RNase L levels were found to depend both on interferon (IFN) treatment and on cell growth conditions. Treatment of murine L cells and EAT cells with 100–2,000 IRU IFNβ or IFNγ resulted in a similar 2–4‐fold increase in the levels of RNase L when cells were present at low density. The levels of RNase L were also shown to increase 2–3‐fold as cells approached saturation density. Serum‐starved cells also displayed relatively high levels of RNase L. RNase L levels in cells maintained at high cell density did not change appreciably following treatment with IFNβ or IFNγ. Regulation of RNase L levels by cell growth conditions as well as by IFNβ or IFNγ treatment suggests that RNase L may play an important role in regulating the levels of cellular mRNAs as well as acting to degrade viral RNAs.