Transcriptional Organization and Regulation of Magnetosome Operons in Magnetospirillum gryphiswaldense

Abstract

Genes involved in magnetite biomineralization are clustered within the genomic magnetosome island of Magnetospirillum gryphiswaldense. Their transcriptional organization and regulation were studied by several approaches. Cotranscription of genes within the mamAB, mamDC, and mms clusters was demonstrated by reverse transcription-PCR (RT-PCR) of intergenic regions, indicating the presence of long polycistronic transcripts extending over more than 16 kb. The transcription start points of the mamAB, mamDC, and mms operons were mapped at 22 bp, 52 bp, and 58 bp upstream of the first genes of the operons, respectively. Identified ؊10 and ؊35 boxes of the P mamAB , P mamDC , and P mms promoters showed high similarity to the canonical 70 recognition sequence. The transcription of magnetosome genes was further studied in response to iron and oxygen. Transcripts of magnetosome genes were detected by RT-PCR both in magnetic cells grown microaerobically under iron-sufficient conditions and in nonmagnetic cells grown either aerobically or with iron limitation. The presence of transcripts was found to be independent of the growth phase. Further results from partial RNA microarrays targeting the putative magnetosome transcriptome of M. gryphiswaldense and real-time RT-PCR experiments indicated differences in expression levels depending on growth conditions. The expression of the mam and mms genes was down-regulated in nonmagnetic cells under iron limitation and, to a lesser extent, during aerobic growth compared to that in magnetite-forming cells grown microaerobically under iron-sufficient conditions.Magnetic orientation in magnetotactic bacteria is based on the synthesis of magnetosomes, which consist of crystals of magnetite (Fe 3 O 4 ) enclosed within intracytoplasmic vesicles of the magnetosome membrane (MM) (1, 29). The MM consists of a lipid bilayer, which provides spatial and physicochemical control over magnetite biomineralization and has a distinct biochemical composition. In the microaerophilic alphaproteobacterium Magnetospirillum gryphiswaldense, the MM is associated with a characteristic subset of magnetosome membrane proteins (MMPs), which are present in different quantities, with relative abundances between Ͻ1% and Ͼ15% (11). Classes of MMPs include those with presumed functions in the activation of magnetosomes, magnetosome-directed transport of iron, nucleation and control of crystal growth, and the assembly of magnetosome chains (16,26,28,29). The targeting of MMPs to the MM is controlled by an unknown mechanism, but it can be assumed that stoichiometric synthesis of individual constituents is regulated for proper assembly of magnetosomes (29).In M. gryphiswaldense, the MMPs are encoded within a hypervariable 130-kb genomic magnetosome island (MAI) (28,35). In addition to all known magnetosome genes, the MAI contains further genes putatively involved in magnetosome biomineralization and is particularly rich in insertion elements. The mam (magnetosome membrane) and mms (magnetic particle membrane-specific) ge...

Common ancestry of iron oxide- and iron-sulfide-based biomineralization in magnetotactic bacteria

“…These genes not only show similar sequence homologies to others of their type, the organization of these genes show similar short intergenic distances between them and an identical direction of transcription, providing evidence that they are organized as an operon as described for other MTB (Schü bbe et al, 2006). The presence of mam genes and their organization in the Nitrospirae and various classes of the Proteobacteria strongly supports a monophyletic origin for magnetite-based magnetotaxis.…”

Formation of magnetite by bacteria and its application

“…The transcription levels of several genes, which varied in response to the iron and oxygen concentrations in the MSR-1 strain, were analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) and microarray. Although results from the two methods were not consistent in certain areas, seven genes (mgI458, mgI460, mamG, mamD, mamM, mamA and mamB) were determined to be downregulated under the iron-limiting condition (less than 1 mM iron; Schübbe et al 2006). Seven other genes (mgI458, mgI460, mamD, mamH, mamK, mamM and mamA) were downregulated under the aerobic (10 kPa O 2 ) condition in both the methods.…”

Genetic Dissection of the mamAB and mms6 Operons Reveals a Gene Set Essential for Magnetosome Biogenesis in Magnetospirillum gryphiswaldense

“…It has been shown that the regions between and flanking the identified magnetosome operons have no functional relevance for magnetosome biosynthesis in M. gryphiswaldense and M. magneticum (7,18). In M. gryphiswaldense, the mms6, mamGFDC, mamAB, and mamXY operons are transcribed as single polycistronic messengers under the control of the P mms6 , P mamDC , P mamH , and P mamXY promoters, respectively (19,20). A mutant with a deletion of mamGFDC op , encoding the most abundant magnetosome proteins, retained the ability to form magnetic, although smaller and less regular, magnetosomes, while plasmidal overexpression of the entire mamGFDC operon yielded magnetite particles even larger than those produced by the wild type (WT) (21).…”