Bacterial transcription is the process in which a segment of bacterial DNA is copied into a newly synthesized strand of messenger RNA (mRNA) with use of the enzyme RNA polymerase. The process occurs in three main steps: initiation, elongation, and termination; and the end result is a strand of mRNA that is complementary to a single strand of DNA. Generally, the transcribed region accounts for more than one gene. In fact, many prokaryotic genes occur in operons, which are a series of genes that work together to code for the same protein or gene product and are controlled by a single promoter. Bacterial RNA polymerase is made up of four subunits and when a fifth subunit attaches, called the σ-factor, the polymerase can recognize specific binding sequences in the DNA, called promoters. The binding of the σ-factor to the promoter is the first step in initiation. Once the σ-factor releases from the polymerase, elongation proceeds. The polymerase continues down the double stranded DNA, unwinding it and synthesizing the new mRNA strand until it reaches a termination site. There are two termination mechanisms that are discussed in further detail below. Termination is required at specific sites for proper gene expression to occur. Gene expression determines how much gene product, such as protein, is made by the gene. Transcription is carried out by RNA polymerase but its specificity is controlled by sequence-specific DNA binding proteins called transcription factors. Transcription factors work to recognize specific DNA sequences and based on the cells needs, promote or inhibit additional transcription.