As well as understanding how transcription works, it is necessary to know about when it is initiated. The two main methods by which a gene's expression is controlled involve regulatory proteins which are either repressors or activators.
Repressors bind to DNA at or near a promoter site (the location where a repressor binds is called an operator), thereby making it impossible for the ribosomal RNA to bind and begin transcription. However, for every repressor there is an effector which can operate in one of two ways. The first type of effector is called an inducer. In this case the repressor is normally bound to the DNA, but if the effector is present it will bind the repressor changing its shape and its ability to bind the DNA. The repressor breaks away from the DNA and transcription may begin. The second type of effector is called a corepressor, the difference in this case is that the repressor normally doesn't bind the DNA, but the corepressor binds the repressor, changing its shape so that it can bind the DNA and inhibit transcription. This kind of mechanism can be effectively used to control the concentration of a particular protein if the protein is the corepressor for its own repressor. When the protein concentration is low more of it is produced, and as the concentration increases it starts to supress its own synthesis.
Activators are the other type of regulatory protein. These bind to DNA near promoter sites (the location where an activator binds is know as an activator site) and attract RNA polymerase to the region, making initiation of transcription more likely. Sometimes another molecule called an effector is involved, which binds to the activator and increases the chances of the RNA polymerase binding to the activator site.