Now, prokaryotic cells such as bacterial cells can regulate gene expression by using special types of molecules. Special types of proteins known as DNA binding proteins and DNA binding proteins used by prokaryotes come in two categories. We have repressors and we have activators. So during the process of transcription when we transcribe the DNA into RNA the activators can basically activate transcription while the repressors can block the process of transcription and ultimately block the synthesis of proteins that correspond to those genes. Now, our activators at repressive protein molecules can themselves be controlled alisterally by other molecules as we'll see in just a moment. So when we're discussing prokaryotic gene regulation and expression basically the model that describes the regulation of genes in prokaryotic organisms is known as the operon model.