Extrinsic vs. intrinsic noises in phage lambda genetic switch

Noises in biological modeling may be classified into two kinds: intrinsic noise, which derives from the variability in dominant molecular interaction and is responsible for the given phenomenon, and extrinsic noise, which arises from other sources, like fluctuations in the environment and so on. Phage lambda is a simple model organism that exhibits important noisy characteristics. It lives in either lysogenic state or lytic state after infecting a bacterium, that is determined by a genetic switch. The mathematical modeling of this genetic switch typically only considers intrinsic noise, though a previous study by one of present authors suggested the critical role of extrinsic noise. In the present study by comparing theoretical results of phage lambda in lysogeny with experiment data, we first achieve good numerical agreements of five constrains of phage lambda for averaged variables. This success indicates that current dominant molecular agents are right. In addition, we confirm the existence of extrinsic noise in lambda genetic switch and find it surprisingly large. This finding calls for an extension of the current mathematical model to better describe the noises. We also point out some possible sources of extrinsic noise.