Gene Expression Noise in Spatial Patterning: hunchback Promoter Structure Affects Noise Amplitude and Distribution in Drosophila Segmentation

Abstract

Noise is an intrinsic part of biochemical systems such as gene regulation networks. Noisy gene expression has been well documented in populations of single cells, and is likely a key mechanism in evolutionary change. But in developing embryos, cells within a tissue must overcome such variability in order to provide the uniformity required to coordinate multiple events. Reproducibility and determinacy of the spatial protein patterns preceding tissue differentiation is a critical aspect of development. In this study, we use anterior-posterior (AP) segmentation in the fruit fly (Drosophila) to understand how gene regulation dynamics control noise. One of the earliest AP patterning events is the anterior activation of the hunchback (hb) gene by the maternally-derived Bicoid (B_cd) protein gradient. This interaction has been very well characterized, providing the tools for us to develop a stochastic model of hb gene regulation to make predictions about expression noise, and to corroborate these experimentally. For hb, we show that self-regulation is a critical part of controlling noise, and the multiple B_cd binding sites in the hb promoter also enhance pattern reproducibility. To the degree that such features are shared by other genes, these noise-reducing mechanisms may be common to many pattern forming events.

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