Regulatory Pathways So far we have been considering simple regulatory systems with either a single repressor (Lac)or a single activator(Mal). Often genes are regulated by a more complicated set of regulatory steps, which together can be thought of as a regulatory pathway. Although there are good methods that can be used to determine the order of steps in a regulatory pathway(as will be discussed shortly), it is usually difficult at first to tell whether a given component identified by mutation is acting directly on the dna of the regulated gene or whether it is acting at a step upstream in a regulatory pathway. For example, it will often be the case that a recessive trans-acting mutation that causes constitutive expression is not an actual repressor protein, but a protein acting upstream in a regulatory pathway in such a way that the net effect of this proteins is to cause repression of gene function. The best way to represent this situation is to call the gene product a negative regulator and to reserve the term repressor for cases in which we know that the protein actually shuts off transcription directly by binding to an operator site. Similarly, the best way to represent a gene defined by a recessive, trans-acting mutation that causes uninducible expression as a positive activator until more specific information can be obtained about whether or not the gene product directly activates transcription. The diagrams to be used are shown below negative positive requlator activator zyme enzyme An important note about interpreting such diagrams is that the arrow or blocking symbol do not necessarily imply direct physical interaction simply that the negative regulator or positive activator have a net negative or positive effect, respectively, on gene expressionRegulatory Pathways So far we have been considering simple regulatory systems with either a single repressor (Lac) or a single activator (Mal). Often genes are regulated by a more complicated set of regulatory steps, which together can be thought of as a regulatory pathway. Although there are good methods that can be used to determine the order of steps in a regulatory pathway (as will be discussed shortly), it is usually difficult at first to tell whether a given component identified by mutation is acting directly on the DNA of the regulated gene or whether it is acting at a step upstream in a regulatory pathway. For example, it will often be the case that a recessive trans-acting mutation that causes constitutive expression is not an actual repressor protein, but a protein acting upstream in a regulatory pathway in such a way that the net effect of this proteins is to cause repression of gene function. The best way to represent this situation is to call the gene product a negative regulator and to reserve the term repressor for cases in which we know that the protein actually shuts off transcription directly by binding to an operator site. Similarly, the best way to represent a gene defined by a recessive, trans-acting mutation that causes uninducible expression as a positive activator until more specific information can be obtained about whether or not the gene product directly activates transcription. The diagrams to be used are shown below. negative – positive regulator activator enzyme + enzyme An important note about interpreting such diagrams is that the arrow or blocking symbol do not necessarily imply direct physical interaction simply that the negative regulator or positive activator have a net negative or positive effect, respectively, on gene expression