Catabolite repressor/activator (Cra), formerly known as fructose repressor FruR. It is a transcription factor found in enteric bacteria such as Escherichia coli and Salmonella. Its function is to control the use of carbon sources by bacteria by repressing some genes and activating others.
Function Cra can act both as a repressor and an activator of gene expression. Its main function is to regulate genes that participate in bacterial metabolism and help bacteria adapt to the nutrients available.
Role in carbon metabolism Cra regulates the flow of carbon in various metabolic pathways. It down-regulates some genes that participate in glycolysis and up-regulates genes involved in other metabolic pathways, such as the tricarboxylic acid cycle and gluconeogenesis.
Effector binding Effectors are the small sugar molecules that control the activity of Cra. The protein changes shape and releases the DNA it was bound to after an effector binds to Cra, so its target genes are no longer repressed or activated. Over the years, fructose-1-phosphate (F1P) and fructose-1,6-bisphosphate (FBP) were thought to act as effectors of Cra. One study suggested that by responding to FBP levels in the cell, Cra works as a sensor of how fast glycolysis is running. However, later research found that FBP does not directly control it and F1P is the real effector of Cra.
Comparison with other systems Bacteria use different mechanisms for regulating carbon metabolism. In enteric bacteria, the main system uses the cAMP receptor protein (CRP) and Cra works alongside it, while in other bacteria, including most Firmicutes, a protein called CcpA is used.
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