Final answer:
The σ subunit of prokaryotic RNA polymerase is responsible for promoter recognition and is not a permanent part of the RNA polymerase complex; it dissociates after the initiation of transcription.
Step-by-step explanation:
The sigma (σ) subunit of prokaryotic RNA polymerase plays a crucial role in the initiation of transcription. It is responsible for recognizing promoter sequences in the DNA, which are specific regions where transcription begins. Once transcription has commenced, the σ subunit dissociates from RNA polymerase, allowing the core enzyme to continue the process of elongation, where the RNA strand is synthesized. This means that the σ subunit is not a permanent part of the RNA polymerase complex, but rather a temporary factor that guides the enzyme to the correct transcription start site. Moreover, the σ subunit can be recycled and used to initiate transcription at multiple sites, and it does not decrease, but rather increases, the affinity of RNA polymerase for DNA by helping it bind to the correct segment of the genome.
Based on this information, the characteristic that is NOT a feature of the σ subunit is that it is a permanent part of the RNA polymerase complex. The correct choice is, therefore, 'It is a permanent part of the complex.' This differentiates the σ subunit's role from the core subunits of the RNA polymerase, which assemble at each gene each time transcription is initiated and disassemble once it is complete.