Final answer:
Positive and negative regulator proteins that modulate transcription in eukaryotes by binding to DNA are called transcription factors. They can act as activators or repressors to control gene expression and respond to the cell's needs.
Step-by-step explanation:
Positive and negative regulator proteins that bind to specific regions of the DNA and stimulate or inhibit transcription in eukaryotes are known as transcription factors. These proteins play critical roles in gene expression regulation by serving as either activators or repressors. Activators are transcription factors that enhance the interaction between RNA polymerase and the promoter, promoting transcription initiation. Conversely, repressors inhibit transcription progression by blocking the path of RNA polymerase on the DNA strand.
Transcription factors can also bind to distant DNA sequences known as enhancers, contributing to increased levels of transcription by facilitating the looping back of these sequences to the gene's promoter. The interplay between these proteins and the binding sites — including the promoter, enhancers, and other regulatory elements — allows a cell to precisely control which genes are expressed and to what extent, in response to environmental cues and the cell's needs.