Restoration of the Functional nif Gene Cluster by Complex Recombination Events during Heterocyst Development in the Nitrogen-Fixing Cyanobacterium Calothrix sp. NIES-4101.
Kazuma Uesaka, Mari Banba, Sotaro Chiba, Yuichi Fujita
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引用次数: 0
Abstract
In the genome of the heterocystous cyanobacterium Calothrix sp. NIES-4101 (NIES-4101), the four genes essential for nitrogen fixation (nifB, nifH, nifD and nifK) are highly fragmented into 13 parts in a 350-kb chromosomal region, and four of these parts are encoded in the reverse strand. Such a complex fragmentation feature makes it difficult to restore the intact nifBHDK genes by the excision mechanism found in the nifD gene of the Anabaena sp. PCC 7120 heterocyst. To examine the nitrogen-fixing ability of NIES-4101, we confirmed that NIES-4101 grew well on a combined nitrogen-free medium and showed high nitrogenase activity, which strongly suggested that the complete nifBHDK genes are restored by a complex recombination process in heterocysts. Next, we resequenced the genome prepared from cells grown under nitrogen-fixing conditions. Two contigs covering the complete nifHDK and nifB genes were found by de novo assembly of the sequencing reads. In addition, the DNA fragments covering the nifBHDK operon were successfully amplified by PCR. We propose that the process of nifBHDK restoration occurs as follows. First, the nifD-nifK genes are restored by four excision events. Then, the complete nifH and nifB genes are restored by two excision events followed by two successive inversion events between the inverted repeat sequences and one excision event, forming the functional nif gene cluster, nifB-fdxN-nifS-nifU-nifH-nifD-nifK. All genes coding recombinases responsible for these nine recombination events are located close to the terminal repeat sequences. The restoration of the nifBHDK genes in NIES-4101 is the most complex genome reorganization reported in heterocystous cyanobacteria.
期刊介绍:
Plant & Cell Physiology (PCP) was established in 1959 and is the official journal of the Japanese Society of Plant Physiologists (JSPP). The title reflects the journal''s original interest and scope to encompass research not just at the whole-organism level but also at the cellular and subcellular levels.
Amongst the broad range of topics covered by this international journal, readers will find the very best original research on plant physiology, biochemistry, cell biology, molecular genetics, epigenetics, biotechnology, bioinformatics and –omics; as well as how plants respond to and interact with their environment (abiotic and biotic factors), and the biology of photosynthetic microorganisms.