Core RNA polymerase consists of five of the six subunits of RNA polymerase. This is the form that adds nucleotides to synthesize the RNA transcript.  
The holo form of RNA polymerase consists of all six subunits with the sixth being the sigma subunit. This allows for specificity of RNA polymerase to interact with different promoters of genes. The holo form is present during initiation, but elongation begins when the sigma subunit dissociates from RNA polymerase.

University of Wisconsin - Madison

RNA polymerase adds nucleotides to synthesize RNA transcripts in transcription. It does not require the RNA primer for initiation of transcription like DNA polymerase. 
RNA Polymerase interacts with the -35 and -10 sequence relative to the start of DNA transcription (+1) to begin initiating transcription. 

RNA Polymerase in Prokaryotes

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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