Primary gene induction or repression in eukaryotes does not require de novo protein synthesis, suggesting the involvement of posttranslational modifications. Since many different types of stimuli that affect gene expression also lead to the activation of protein kinases, it is likely that transcription factor function will be directly regulated by phosphorylation. Even though other types of posttranslational modifications will undoubtedly be important in regulating transcription factor function, this review is strictly focused on the role of phosphorylation, since this is the best studied modification system (for other recent reviews see Bohmann, 1990; Jackson, 1992). Three main levels of regulation modulate transcription factor activity (see Table 1). First, transcription factors can be sequestered in the cytoplasm and rendered inactive through lack of access to their target sequences. Phosphorylation of the factor itself or a …