RESUMO
The continuous changes in incident solar light occurring during the day oblige oxyphototrophs, such as the marine prokaryote Prochlorococcus, to modulate the synthesis and degradation rates of their photosynthetic components finely. How this natural phenomenon influences the diel expression of photosynthetic genes has never been studied in this ecologically important oxyphotobacterium. Here, the high light-adapted strain Prochlorococcus sp. PCC 9511 was grown in large-volume continuous culture under a modulated 12 h-12 h light-dark cycle mimicking the conditions found in the upper layer of equatorial oceans. The pcbA gene encoding the major light-harvesting complex showed strong diel variations in transcript levels with two maxima, one before the onset of illumination and the other near the end of the photoperiod. In contrast, the mRNA level of psbA (encoding the reaction centre II subunit D1), the monocistronic transcript of psbD (encoding D2) and the dicistronic transcript of psbDC were all tightly correlated with light irradiance, with a minimum at night and a maximum at noon. The occurrence of a second peak during the dark period for the monocistronic transcript of psbC (encoding one of the PS II core Chl a antenna proteins) suggested the involvement of post-transcriptional regulation. Differential expression of the external antenna and core genes may constitute a mechanism of regulation of the antenna size to cope with the excess photon fluxes that Prochlorococcus cells experience in the upper layer of oceans around midday. The 5' ends of all transcripts were mapped, and a conserved motif, 5'-TTGATGA-3', was identified within the putative psbA and pcbA promoters.
Assuntos
Cianobactérias/genética , Cianobactérias/fisiologia , Escuridão , Regulação Bacteriana da Expressão Gênica , Genes Bacterianos/genética , Luz , Fotossíntese/genética , Proteínas de Bactérias/genética , Sequência de Bases , Cianobactérias/citologia , Cianobactérias/crescimento & desenvolvimento , Citometria de Fluxo , Dosagem de Genes , Óperon/genética , Regiões Promotoras Genéticas/genética , RNA Bacteriano/análise , RNA Bacteriano/genéticaRESUMO
The cell cycle of the chlorophyll b-possessing marine cyanobacterium Prochlorococcus is highly synchronized under natural conditions. To understand the underlying molecular mechanisms we cloned and sequenced dnaA and ftsZ, two key cell cycle-associated genes, and studied their expression. An axenic culture of Prochlorococcus sp. strain PCC 9511 was grown in a turbidostat with a 12 h-12 h light-dark cycle for 2 weeks. During the light periods, a dynamic light regimen was used in order to simulate the natural conditions found in the upper layers of the world's oceans. This treatment resulted in strong cell cycle synchronization that was monitored by flow cytometry. The steady-state mRNA levels of dnaA and ftsZ were monitored at 4-h intervals during four consecutive division cycles. Both genes exhibited clear diel expression patterns with mRNA maxima during the replication (S) phase. Western blot experiments indicated that the peak of FtsZ concentration occurred at night, i.e., at the time of cell division. Thus, the transcript accumulation of genes involved in replication and division is coordinated in Prochlorococcus sp. strain PCC 9511 and might be crucial for determining the timing of DNA replication and cell division.
Assuntos
Proteínas de Bactérias/biossíntese , Ciclo Celular/genética , Cianobactérias/genética , Proteínas do Citoesqueleto , Proteínas de Ligação a DNA/biossíntese , Proteínas de Escherichia coli , Periodicidade , Genes Bacterianos , Genoma Bacteriano , Biologia Marinha , Proteínas de Membrana , Dados de Sequência Molecular , FotoperíodoRESUMO
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.
Assuntos
Cianobactérias/genética , Genes Bacterianos , Luz , Complexo de Proteínas do Centro de Reação Fotossintética/genética , Sequência de Aminoácidos , Sequência de Bases , Cianobactérias/fisiologia , Primers do DNA , Complexos de Proteínas Captadores de Luz , Dados de Sequência Molecular , Complexo de Proteínas do Centro de Reação Fotossintética/química , Homologia de Sequência de AminoácidosRESUMO
Data obtained by pulsed field gel electrophoresis revealed for Prochlorococcus marinus CCMP 1375 a genome size of 1.81+/-0.04 Mbp. This value is significantly smaller than for all other cyanobacteria investigated so far. The absence of an intron in the gene for tRNA(Leu)(UAA), which otherwise is widespread among cyanobacteria, and the additional finding that the ribosomal operon exists as a single copy suggest that the deletion of non-essential sequences played a major role in the evolution of P. marinus. A small genome may have been advantageous in the adaptation to very oligotrophic marine conditions.
