Two general mechanisms that would result in CN expression being greater than C+N. (a) Carbon (C) and CN regulatory elements are independent and do not interact. The data do not allow us to rule out the possibility that the C-element is inactive in the presence of CN and that the CN-element alone results in more expression than the C-element. (b) CN and carbon regulation are dependent. The increase in expression due to CN requires two interacting cis elements, one of which is a C-element and the other a nitrogen-dependent enhancer of carbon regulation (NDE).