Biology Reference
In-Depth Information
metal homeostasis is crucial for the ecological success of cyanobacteria, trace metal
bio-uptake is strictly regulated by a number of metal-sensor proteins and regula-
tory proteins that often also contain metals. This chapter discusses functional studies
undertaken to-date from a genomic point of view, as well as the main structural and
mechanistic insights into the major families of metalloregulators in cyanobacteria.
Reverse genetics, transcriptomics and other assays used for the identification of metal-
regulated genes reveal interesting connections between metabolic networks and
interactivity between major regulons. These data provide a better understanding of
cyanobacterial physiology including maintenance of metal homeostasis, strategies to
deal with different stresses and the basis of cyanotoxicity.
1. INTRODUCTION
Transition metals are essential components of all living cells. They
act as main cofactors for oxidation-reduction reactions in electron transfer
chains, in hydrolytic and acid-base chemistry and are key structural ele-
ments that stabilize protein fold. Cyanobacterial metabolism relies on the
activity of many enzymes and other proteins that contain metal-rich cofac-
tors that are absent in nonphotosynthetic organisms. Most of these micro-
nutrients play key roles in or are associated to photosystems, for example,
manganese in the water-splitting oxygen-evolving complex, magnesium in
chlorophyll and copper in plastocyanin. Besides, biological nitrogen fixa-
tion needs a large amount of metals, such as the nitrogenase complex with
molybdenum (or vanadium) and iron. For these reasons, cyanobacterial
metal requirements are greater than in other prokaryotes. In particular, the
high content of iron present in the machineries involved in photosynthesis
and nitrogen assimilation makes cyanobacteria highly dependent on iron,
whose needs are one order of magnitude larger than those in heterotrophic
bacteria (Shcolnick & Keren, 2006).
During their evolution, cells develop a broad network of metalloregula-
tory proteins involved in transport, metal trafficking and metal homeostasis
and resistance to deficiency, as well as an efficient cross-talk with other
regulatory networks. In fact, the effectiveness of these systems confers on
organisms an important adaptive advantage, which is very clear in the case
of iron metabolism ( Straus, 1994 ). In many cases, the ability of a micro-
organism to capture and incorporate metals determines its ecological success.
In fact, cyanobacteria developed, during evolution, very efficient mechanisms
for maintaining metal homeostasis.
Metalloregulation in cyanobacteria is mainly carried out by metal-
sensor proteins and regulatory proteins containing metals. In recent years,
 
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