The correct answer is: transcription factors.
Gene expression is controlled (regulated) on different levels (DNA structure, transcription, RNA processing, post-translational modification. Gene expression is regulated in order to increase or decrease the production of specific gene products.
Transcription factors can enhance the interaction between RNA polymerase and a DNA sequence- promoter, encouraging the expression of the gene. Such transcription factors are called activators. Otherwise, when the gene expression is inhibited, factors are called repressors and they bind to sequence –operator.
