Translation of galE and coordination of galactose operon expression in Escherichia coli: effects of insertions and deletions in the non-translated leader sequence.

Using gene-manipulation techniques, we made a set of short insertions and deletions in the Escherichia coli galactose operon between the transcription start site and the Shine-Dalgarno sequence of the first gene of the operon, galE. Translation initiation is severely reduced when the distance between the 5' end of the message and the Shine-Dalgarno sequence drops below 15 bases. Transcription of the gal operon can start at two distinct sites, S1 and S2, separated by 5 bp, situated 16 and 21 bp upstream of the galE Shine-Dalgarno sequence, respectively. When transcription starts at S2, gal operon expression is discoordinate as the galE gene is better translated than promoter-distal genes. Here we report that gal operon expression is discoordinate even when message starting at S2 is shortened. This shows that the better translation of galE from transcripts starting at S2 is not simply due to the fact that they are longer than transcripts starting at S1.