Expression of Escherichia coli infC: identification of a promoter in an upstream thrS coding sequence

infC, the gene which codes for translation initiation factor 3, is situated in a cluster in the genome of Escherichia coli with genes for several other components of the translation apparatus. Only three nucleotides separate the termination codon of thrS from the initiation codon of infC. This implies that infC is either cotranscribed with thrS from a thrS promoter or that the transcriptional signals for infC are embedded within the upstream thrS coding region. In the present work, several plasmids have been constructed which encompass infC and various amounts of the upstream thrS sequence. The ability of the plasmid DNA, or derived restriction fragments, to direct the synthesis of initiation factor 3 was tested in an in vitro DNA-dependent coupled transcription-translation system and in plasmid-transformed maxicells. The results indicate that initiation factor 3 is synthesized in the absence of the thrS promoter. A promoter whose presence is sufficient for the expression of infC has been localized to an 89-base-pair region which lies 178 to 267 base pairs upstream of the infC initiation codon. S1 nuclease mapping of in vivo transcripts confirms that a transcription initiation site is located in this region. These studies demonstrate that infC can be transcribed from a promoter within the upstream thrS coding sequence.