Untersuchungen zur molekularen Organisation und Regulation der mam-Gene in Magnetospirillum gryphiswaldense MSR-1

Abstract

The biomineralization of the magnetosomes in magnetotactic bacteria (MTB) is governed by coordinated cellular and physico-chemical processes, which are controlled by the magnetosome membrane proteins (MMP), but poorly understood on the biochemical and molecular level. In this work the genomic and transcriptional organization of the mam and mms genes encoding these MMPs in Magnetospirillum gryphiswaldense were investigated. A close genetic linkage of the mam genes could be determined. The colocalization of all magnetosome genes, the high abundance of insertion sequence elements, and its high genetic instability lead to the discovery of a magnetosome island (MAI) of approximately 130 kb. Spontaneous non-magnetic mutants exhibiting deletions or rearrangements within the MAI occurred frequently under conditions of prolonged storage and oxygen stress. One of these non-magnetic mutants, designated strain MSR-1B, harbours a deletion of 40 kb comprising all magnetosome genes. Long polycistronic transcripts could be confirmed by RT-PCR for three major gene operons harbouring magnetosome genes. The transcriptional start points were determined 22 bp, 52 bp, and 58 bp upstream of the mamAB, mamDC and mms operons, respectively. The promoter regions showed homology to the consensus sequence of the vegetative sigma factor sigma 70. The magnetosome genes were expressed both under magnetic and non-magnetic growth conditions, but showed a down-regulation under iron-limited and aerobic conditions as revealed by microarray and real-time RT-PCR. The mamK gene product, which has homology with actin-like MreB proteins showed a linear intracellular localization as revealed by a GFP-MamK fusion, and a cytoplasmic localization was confirmed by 2D gel electrophoresis and Western blots. Therefore a putative role in the formation of the magnetosome chain was suggested.