Work with chloroplast RNA polymerase from various plant species has established the presence of transcription factors that either act on the DNA template or on other polypeptides of the transcription complex. The first example was a 26-kDa polypeptide from maize chloroplasts, referred to as S-factor (1 ), which was shown to stimulate the rate of transcription from chimeric supercoiled templates. In its presence, maize chloroplast RNA polymerase was found to transcribe chloroplast sequences preferentially as compared to vector sequences, if the plasmid template was in a supercoiled form. The S-factor appears to act generally, rather than on a specific set of chloroplast genes (2 ). Another factor described is a 115-kDa DNA-binding protein from pea chloroplasts, which was shown to interact with a 5′ region of a specific chloroplast gene (3 ). Further examples include a DNA-binding polypeptide of approx 70 kDa found associated with partially purified RNA polymerase from spinach chloroplasts (4 ) and several DNA-binding polypeptides that copurified with chloroplast RNA polymerase transcription complexes from mustard (5 ) and tobacco (6 ).