Bypassing the Need for the Transcriptional Activator EarA through a Spontaneous Deletion in the BRE Portion of the Operon Promoter in .

In , the euryarchaeal archaellum regulator A (EarA) is required for the transcription of the operon, which is comprised of a series of genes which encode most of the proteins needed for the formation of the archaeal swimming organelle, the archaellum. In mutants deleted for (Δ), there is almost undetectable transcription of the operon, Fla proteins are not synthesized and the cells are non-archaellated. In this study, we have isolated a spontaneous mutant of a Δ mutant in which the restoration of the transcription and translation of the operon (using , the second gene of the operon, as a reporter), archaella formation and swarming motility were all restored even in the absence of EarA. Analysis of the DNA sequence from the promoter of this spontaneous mutant revealed a deletion of three adenines within a string of seven adenines in the transcription factor B recognition element (BRE). When the three adenine deletion in the BRE was regenerated in a stock culture of the Δ mutant, very similar phenotypes to that of the spontaneous mutant were observed. Deletion of the three adenines in the promoter BRE resulted in the mutant BRE having high sequence identity to BREs from promoters that have strong basal transcription level in and . These data suggest that EarA may help recruit transcription factor B to a weak BRE in the promoter of wild-type cells but is not required for transcription from the promoter with a strong BRE, as in the three adenine deletion version in the spontaneous mutant.