Role of the fnrL gene in photosystem gene expression and photosynthetic growth of Rhodobacter sphaeroides 2.4.1

Abstract

Anoxygenic photosynthetic growth of Rhodobacter sphaeroides 2.4.1 requires a functional fnrL gene, which encodes the anaerobic regulator, FnrL. Using transcriptional fusions to the puc operon in which the upstream FNR consensus-like sequence is either present or absent, we obtained results that suggest that FnrL has both a direct and an indirect role in puc operon expression. In addition to FnrL, several other factors, including the two-component Prr regulatory system and the transcriptional repressor PpsR, are known to mediate oxygen control of photosynthesis gene expression in this organism. Therefore, we examined the relationship between FnrL and these other regulatory elements. Our results indicate that while mutations of prr or ppsR can lead to an increase in expression of some photosynthesis genes under aerobic and anaerobic conditions, regardless of the presence or absence of FnrL, there remains an absolute requirement for a functional fnrL gene for photosynthetic growth. We examined the potential role(s) of FnrL in photosynthetic growth by considering several target genes which may be required for this growth mode.

FIG. 1

Analysis of puc operon expression in wild-type and FnrL⁻ strains using puc::lacZ transcription fusion plasmids. (A) Schematic diagram of the relevant features of the puc::lacZ transcription fusion plasmids, pCF200 and pCF250. (B) β-Galactosidase activity versus time, following a shift from 30 to 2% oxygen, of cultures of R. sphaeroides 2.4.1(pCF200) (•), JZ1678(pCF200) (X), 2.4.1(pCF250) (▪), and JZ1678(pCF250) (□).

FIG. 2

Comparison of the relevant DNA sequences of mutant allele prrB78* and wild-type prrB, together with their predicted protein sequences.

FIG. 3

Schematic diagram of the growth of wild-type and FnrL⁻ mutant strains of R. sphaeroides 2.4.1 bearing various plasmids following a shift from aerobic (30% oxygen) to photosynthetic (anaerobic, 10 W/m²) conditions. For each strain, all of at least three samples analyzed showed growth patterns similar to the examples presented here. (A) Growth of wild-type 2.4.1 cells bearing plasmids pRK415 (□), pUI1649 (▪), pUI1650 (○), and pUI1650* (•). (B) Growth of FnrL⁻ mutant strain JZ1678 bearing plasmids pRK415 (□), pUI1649 (▪), and pUI1650* (○). (C) Extended growth of wild-type 2.4.1(pRK415) (□) and JZ1678(pRK415) (▪).