Many deep-sea bacteria have evolved specialized adaptations for life at cold temperatures and high pressures. A locus required for both psychro- and baro-adaptation in the psychrophilic, moderate barophile, Photobacterium species strain SS9 was identified among SS9 transposon mutants. DNA sequence analysis of this locus identified four complete open reading frames (ORFs), which appear to comprise an operon, and a fifth incomplete ORF. All transposon insertions isolated are in ORF3. Extensive sequence similarity exists between the translation products of ORFs 1-3 and a collection of gene products proposed to include alternative RNA polymerase sigma factors and modifiers of sigma-factor activity involved in extracytoplasmic sensing and regulation. Based on the similarity between ORF1 and Escherichia coli rpoE, we have tentatively designated this locus the rpoE locus. SS9 rpoE locus ORF3 insertion mutants showed altered abundances of numerous outer membrane proteins and were both baro- and psychro-sensitive. ORF3 mutant revertants that displayed enhanced high-pressure growth also displayed concomitant enhanced low-temperature growth. Most of these revertants possessed DNA rearrangements at the site of the transposon insertion, further demonstrating the importance of the rpoE locus to high-pressure and cold-temperature growth. Complementation analyses indicated that ORF3 functions in OMP synthesis regulation while ORF4 is required for baro- and psychro-adaptation.