Specificity of siderophore-mediated transport of iron in rhizobia

The trihydroxamate siderophore, hydroxamate K, has been purified from culture filtrates of iron-deficient Rhizobium leguminosarum biovar viciae MNF710. The iron complex has a molecular weight of 828 and an absorption maximum at 443 nm (_M=1510). ⁵⁵Fe complexed to purified hydroxamate K was taken up by MNF710, its hydroxamate-negative mutant MNF7102 and Rhizobium leguminosarum biovar trifolii WU95 via an iron-regulated transport system, but Rhizobium meliloti U45 failed to take up the iron-siderophore complex under any conditions. A similar pattern of iron uptake was observed with ferrioxamine B. MNF710, MNF7102, U45 and WU95 all transported ⁵⁵Fe-ferrichrome but only the first three strains took up ⁵⁵Fe-ferrichrome A. All these ⁵⁵Fe-trihydroxamate uptake systems were ironregulated in MNF710, MNF7102 and WU95. In contrast, uptake of ⁵⁵Fe-rhodotorulate, a dihydroxamate, was essentially constitutive in all four organisms. Similarly, uptake of ⁵⁵Fe-citrate and ⁵⁵Fe-nitrilotriacetic acid was constitutive. None of the strains took up ⁵⁵Fe complexed with enterobactin or with pyoverdins from Pseudomonas aeruginosa ATCC15692 (PAO1) and Pseudomonas fluorescens ATCC17400.     

The molecular and cell biology of anion transport by bacteria.

This article summarizes the study of anion exchange mechanisms in bacteria. Along with defining at least two different families of anion exchange, an examination of such carrier-mediated antiport reactions has led to techniques that considerably broaden the scope of biochemical methods for examining membrane proteins. Such advances have been exploited to show that anion exchange itself forms the mechanistic base of an entirely new kind of proton pump, one which may shed light on a variety of bacterial events, including methanogenesis. Perhaps most important, the study of exchange provided the final link in a chain of evidence pointing to a structural 'rhythm' that seems to characterize membrane carriers. These three issues--a biochemical tool, a new proton pump, and a common structural rhythm--are briefly examined in the context of their origins in the analysis of bacterial anion exchange.     

Genetics and molecular biology of methanotrophs.

Over the last 20 years or so, the obligate methane-oxidizing bacteria (methanotrophs) have attracted considerable interest. As they grow on a relatively cheap and abundant carbon source, they appeared ideal organisms for the production of bulk chemicals, single-cell protein and for use in biotransformations. More recently their cooxidation properties have been investigated for bioremediation, including the removal of chlorinated compounds such as trichloroethylene from polluted groundwaters. These studies have resulted in a great deal of information on the physiology and biochemistry of methanotrophs but sadly the molecular biology and genetic studies of these organisms have lagged behind. This has been in part due to the obligate nature of the methanotrophs and the refractory nature of such organisms to conventional genetic analysis. However, the more recent availability of broad-host range plasmids coupled with improvements in molecular biology methods have allowed the development of molecular genetic techniques for methanotrophs. The purpose of this review is to summarize what is known about the genetics and molecular biology of methanotrophs and how this information can be used to complement previous and current biochemical studies on the unique property of these bacteria, i.e. the ability to oxidize methane to methanol. Recent developments in molecular ecology techniques that may be applied to these apparently ubiquitous organism are also considered.