The sid gene promoter (P_sid), which controls expression of the late genes from satellite phage P4, is activated by a unique class of small DNA-binding proteins. The activators from both satellite and helper phages stimulate transcription from P_sid. These activators bind to sites centered at position 55 in all the helper and satellite phage late promoters. P4 P_sid is unique in that it has an additional activator binding site centered at position 18 (site II). We have constructed a mutant of site II that no longer binds activators. Transcription under the control of satellite phage activators is increased by the site II mutation. In contrast, helper phage activators do not show this increase in transcription from P_sid mutated at site II. Competition gel shift analysis reveals that the P4 satellite phage activator, Delta, binds eightfold better to site II than to site I. The products of the sid transcription unit are needed only when a helper phage is present; thus, the satellite phage activators repress transcription until the helper is present to supply a nonrepressing activator.