Saccharomyces cerevisiae prefers glucose as its carbon source, and intercellular glucose acts as an important signaling molecule to regulate many physiological processes, so yeasts have evolved sophisticated signaling pathways to regulate the uptake and metabolism of glucose. The Snf3/Rgt2-Rgt1 (SRR) glucose-sensing pathway enables yeast cells to detect extracellular glucose and transmit an intercellular signal that induces expression of hexose transporter (hxt) genes. The yeast casein kinases (YCKs) are key players in this signaling pathway, and are thought to be responsible for transmission of the glucose signal. The current model of the SRR signaling pathway suggests that binding of glucose to the glucose sensors activates YCKs to phosphorylate the transcriptional co-repressors Mth1 and Std1, targeting them for degradation and thereby relieving repression of Hxt genes. However, we have evidence for an upstream role for YCKs in the SRR signaling pathway: (i) overexpression of Rgt2 rescues glucose signaling in a strain lacking YCK; (ii) the C-terminal tail of Rgt2 is phosphorylated by YCK, and (iii) Rgt2 phosphorylation is necessary for signaling; (iv) Rgt2 phosphorylation promotes interaction of the Mth1 and Std1 co-repressors with the Rgt2 glucose sensor. These results lead us to a model in which the YCKs act upstream of the glucose sensors to phosphorylate the C-terminal tail of the glucose sensors to establish a functional interaction site for the co-repressors, bringing them to the glucose sensor to receive and transmit the glucose signal.