Multiplication of antenna genes as a major adaptation to low light in a marine prokaryote - PubMed (original) (raw)

Multiplication of antenna genes as a major adaptation to low light in a marine prokaryote

L Garczarek et al. Proc Natl Acad Sci U S A. 2000.

Abstract

Two ecotypes of the prokaryote Prochlorococcus adapted to distinct light niches in the ocean have been described recently. These ecotypes are characterized by their different (divinyl-) chlorophyll (Chl) a to Chl b ratios and 16S rRNA gene signatures, as well as by their significantly distinct irradiance optima for growth and photosynthesis [Moore, L. R., Rocap, G. & Chisholm, S. W. (1998) Nature (London) 393, 464-467]. However, the molecular basis of their physiological differences remained, so far, unexplained. In this paper, we show that the low-light-adapted Prochlorococcus strain SS120 possesses a gene family of seven transcribed genes encoding different Chl a/b-binding proteins (Pcbs). In contrast, Prochlorococcus sp. MED4, a high-light-adapted ecotype, possesses a single pcb gene. The presence of multiple antenna genes in another low-light ecotype (NATL2a), but not in another high-light ecotype (TAK9803-2), is demonstrated. Thus, the multiplication of pcb genes appears as a key factor in the capacity of deep Prochlorococcus populations to survive at extremely low photon fluxes.

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Figures

Figure 1

Figure 1

Alignment of the deduced protein sequences from the Prochlorococcus MED4 pcbA gene (M-PcbA) and from Prochlorococcus SS120 pcbA to pcbG genes (S-PcbA to S-PcbG). Identical residues are shown in white type on a black background. Black type on gray squares indicates that the percentage of conserved residues is >60% (i.e., at least 5 of 8 sequences). Predicted membrane-spanning regions (MSR) are indicated by arrows, and stars mark putative Chl-binding residues.

Figure 2

Figure 2

Detection of antenna gene transcripts by Northern blotting from Prochlorococcus MED4 (pcbA) and SS120 (pcbA to pcbG) strains. Transcripts of pcbF have been indicated by an arrow. The sizes of major RNA molecules are indicated.

Figure 3

Figure 3

Characterization of different low-light (LL)- and high-light (HL)-adapted Prochlorococcus ecotypes grown at 20 μmol Q m−2⋅s−1. (A) Excitation fluorescence spectra of whole cells normalized to the divinyl-Chl a peak (444 nm). The variable height of the second peak, mainly monovinyl-Chl b (468 nm) for NATL2a and divinyl-Chl b (478 nm) for the other strains, indicates the widely different Chl b to Chl a ratios between strains. (B) Pattern of amplified pcb fragments obtained on polyacrylamide gels after cleavage by _Hae_III. Lane M, size markers.

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