Differentiation-dependent mechanisms of transcriptional regulation of the catalytic subunit of phosphorylase kinase.

The amount of phosphorylase kinase in skeletal muscle is exquisitely sensitive to developmental signals such as differentiation and innervation, and is clearly regulated in such a manner so as to always maintain the gamma catalytic subunit under the control of its regulatory alpha, beta and gamma subunits. To identify how the transcription of the gamma subunit is regulated, we have analysed 3.8 kb of the upstream regulatory region using a luciferase reporter system. A complex sequence of interdependent regulations is evident. The gamma catalytic subunit gene contains two inhibitory controls with very dominant features. Also evident are an array of multiple positive regulatory elements, prominent amongst which are four E-boxes, of which two are downstream, one is upstream and one is in the middle of the CAAT-TATA core promoter. Differentiation-dependent positive regulation arises as a consequence of both E-box regulation and the activation of at least one other regulatory element. The primary mode of transcriptional regulation of the gamma catalytic subunit gene appears to occur by the relief of regulation of an otherwise default inhibitory status. It is noteworthy that such a mode of regulation mirrors the regulation of the enzymic activity of many protein kinases, including phosphorylase kinase. With phosphorylase kinase, both its transcriptional regulation as well as the regulation of the protein itself, are primed to maintain the gamma catalytic subunit either unexpressed or inactivate respectively, until a positive signal occurs to override an otherwise dominant default inhibitory condition.