Effect of oxygen on translation and posttranslational steps in expression of photosynthesis genes in Rhodobacter capsulatus

Abstract

The formation of the photosynthetic apparatus in Rhodobacter capsulatus is regulated by oxygen tension. Previous studies have shown a regulatory effect of oxygen on the transcription of photosynthesis genes and on the stability of certain mRNA segments. Here we show that oxygen affects puf and puc gene expression posttranslationally and that this regulation depends on the presence of bacteriochlorophyll. Our data suggest that this posttranslational effect of oxygen on puf and puc expression is due to the primary effect of oxygen on bacteriochlorophyll synthesis or assembly of pigment protein complexes. Oxygen does not affect the rates of translation of puf-encoded proteins.

FIG. 1

Partial physical maps of puf and puc operons and plasmids used in this study. (A) Operons encoding R. capsulatus photosynthetic apparatus pigment binding proteins. mRNA species detectable by Northern hybridization are indicated by arrows with open heads, and transcriptional starts are indicated by arrows with solid heads. Stabilizing mRNA secondary structures are also indicated (by “pins”) for the puf operon. All mRNAs drawn in Fig. 1 originate from processing of the highly unstable primary puf and puc transcripts, which cannot be detected in Northern blots. (B) Transcriptional aph-puf fusion in pRK415 (19); (C) translational aph-puf-lacZ fusions used in this study (the lacZ gene is not drawn to scale); (D) β-Galactosidase activity expressed from the aph-lacZ fusions listed in panel C and relative increase of expression (fold) after shift from high to low oxygen.

FIG. 2

(A) Northern blot analysis of total RNA (8 μg per lane) isolated from wild-type strain 37b4 at different time points after a shift to low oxygen tension with simultaneous addition of rifampin at time point 0. Hybridization was performed with puc- and puf-specific DNA fragments. (B) Autoradiography of the membrane proteins synthesized within 3 min of different time points after shift to low oxygen and simultaneous addition of rifampin at time point 0. (C) Quantification of RC, LHI, and LHII proteins together with their mRNA species (symbolized by circles) from the data shown in panels A and B. The value at time zero (t₀) was set as 1. In panel RC, RC-L (PufL) is symbolized by diamonds and RC-M (PufM) is symbolized by squares. In panels LHI and LHII, the α subunits of the light-harvesting complexes LHI and LHII (PufA and PucA, respectively) are symbolized by diamonds and the β subunits (PufB and PucB, respectively) are symbolized by squares.