Final answer:
Expression vectors require a promoter for RNA polymerase binding and a proper reading frame for protein synthesis. The promoter initiates transcription, while the reading frame ensures the correct sequence for the resulting protein.
Step-by-step explanation:
Expression vectors must contain genes coding for a promoter to permit RNA polymerase to initiate transcription and a reading frame to allow translation. A promoter is a DNA sequence to which RNA polymerase and associated factors bind and initiate transcription. This sequence is typically found upstream of the gene and is necessary for the polymerase to recognize the start of a gene. The reading frame refers to the sequence of triplet codons in the mRNA that specify a particular protein. This sequence must be in the correct order for the protein to be synthesized properly; a shift by even a single nucleotide can abolish the synthesis of the correct protein.
The complex of transcription factors and RNA polymerase bound to a promoter is called an initiation complex. These basal factors help position RNA polymerase at the start of the gene, while enhancers and other regulatory proteins control the frequency of transcription.