Osmotic and growth-phase dependent regulation of the eta gene of Staphylococcus aureus: a role for DNA supercoiling

Transcriptional fusions were constructed between the promoter for the epidermolytic toxin A (eta) gene of Staphylococcus aureus and the luxAB and xylE reporter gene systems. The expression of the fusion products was found to be dependent upon the accessory gene regulator (agr) locus and was observed to increase significantly during the transition from the exponential to the stationary phase of growth. Furthermore the expression of the eta gene promoter was found to be osmotically regulated, with the expression levels of the eta fusions being inversely related to the osmolyte levels. The ability of environmental factors to influence DNA topology (and thence gene expression) was investigated. High osmolarity (0.7 M NaCl) resulted in an increase in the degree of negative supercoiling of plasmid DNA in the S. aureus strain 8325-4 (Agr+) but not in strain ISP546 (Agr-). Furthermore the eta promoter was strongly induced in S. aureus cultures grown in the presence of sub-inhibitory concentrations of novobiocin, a DNA gyrase inhibitor. However this induction was independent of agr, suggesting that the eta promoter is subject to both agr-dependent (osmolarity, growth phase) and-independent (DNA topology) regulatory processes.