The start sites of the araC and araBAD gene messenger of E. coli were located by transcription in vitro from short DNA fragments, by high magnification electron microscopy and by genetic mapping. Transcription for these messengers proceeds in opposite directions from the start sites that are 150 base pairs apart. Transcription from the araBAD promoter requires araC protein plus arabinose and CAP protein plus cyclic AMP. In the experiments performed in vitro, inducing the araBAD promoter represses activity of the araC promoter.