bop gene cluster expression in bacteriorhodopsin-overproducing mutants of Halobacterium halobium

mRNA levels from the bop (bacterio-opsin), brp (bacterio-opsin-related protein), and bat (bacterio-opsin activator) genes in wild-type Halobacterium halobium and two bacteriorhodopsin-overproducing mutants (ET1001 and II-7) were quantitated under conditions in which oxygen levels were steadily depleted and then cultures were either kept in the dark or exposed to light. All three strains showed similar responses to depleted oxygen tensions and the lack of light: bop gene cluster transcript levels first increased in response to steadily declining oxygen, and once oxygen was depleted, transcript levels decreased and became undetectable within 20 to 40 h. In contrast, each strain responded differently to conditions of depleted oxygen and the presence of light. In the wild-type strain, bop gene cluster transcript levels increased 2.4- to 9.2-fold above the highest levels obtained in the dark. In mutant ET1001, bop gene cluster transcript levels did not increase above the highest levels obtained in the dark. In mutant II-7, bop and brp transcript levels did not increase above the highest levels obtained in the dark, but bat transcript levels increased approximately 5.7-fold. This differing response to identical physiological conditions indicates that the mutations resulting in the bacteriorhodopsin-overproducing phenotype in these two mutants are different.