Which type of protein helps to alter chromatin structure to enhance transcription?

The correct choice highlights the role of activator proteins in enhancing transcription by altering chromatin structure. Activator proteins bind to specific DNA sequences and recruit other co-activators that facilitate changes in the local chromatin environment, making DNA more accessible for transcription machinery. This often involves modifying histones through processes such as acetylation, which loosens the interaction between histones and DNA, promoting an open chromatin configuration conducive to gene expression.

In contrast, histone methyltransferases and histone deacetylases typically function to either add methyl groups (which can sometimes lead to transcriptional repression) or remove acetyl groups from histones, respectively. These actions generally result in tighter packaging of DNA and a more repressive chromatin state, which does not enhance transcription. Similarly, repressor proteins are involved in inhibiting transcription by binding to specific sites and recruiting factors that promote a closed chromatin structure. Thus, activator proteins are essential for making chromatin more accessible to the transcription machinery and facilitating gene expression.