Deletion of a 55-kilobase-pair DNA element from the chromosome during heterocyst differentiation of Anabaena sp. strain PCC 7120

Abstract

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 produces terminally differentiated heterocysts in response to a lack of combined nitrogen. Heterocysts are found approximately every 10th cell along the filament and are morphologically and biochemically specialized for nitrogen fixation. At least two DNA rearrangements occur during heterocyst differentiation in Anabaena sp. strain PCC 7120, both the result of developmentally regulated site-specffic recombination. The first is an l1-kilobase-pair (kb) deletion from within the 3' end of the nifD gene. The second rearrangement occurs near the nifS gene but has not been completely characterized. The DNA sequences found at the recombination sites for each of the two rearrangements show no similarity to each other. To determine the topology-of the rearrangement near the nitS gene, cosmid libraries of vegetative-cell genomic DNA were constructed and used to clone the region of the chromosome involved in the rearrangement. Cosmid clones which spanned the DNA separating the two recombination sites that define the ends of the element were obtained. The restriction map of this region of the chromosome showed that the rearrangement was the deletion of a 55-kb DNA element from the heterocyst chromosome. The excised DNA was neither degraded nor amplified, and its function, if any, is unknown. The 55-kb element was not detectably transcribed in either vegetative cells or heterocysts. The deletion resulted in placement of the rbcLS operon about 10 kb from the nijS gene on the chromosome. Although the nipD 11-kb and nifS 55-kb rearrangements both occurred under normnal aerobic heterocyst-inducing conditions, only the 55-kb excision occurred in argon-bubbled cultures, indicating that the two DNA rearrangements can be regulated differently.Heterocyst differentiation in the cyanobacterium Anabaena sp. strain PCC 7120 is accompanied by two developmentally regulated genome rearrangements (8,9,14). Unlike most DNA rearrangements in procaryotic organisms, such as the movement of transposable elements and phase variation inversions which occur at very low frequencies (33), rearrangement of the heterocyst genome is tightly regulated by an environmentally induced developmental sequence.Heterocysts are highly specialized, terminally differentiated cells which reduce atmospheric dinitrogen to ammonia and then export the fixed nitrogen to neighboring vegetative cells in the form of glutamine (reviewed in references 12 and 37). Differentiation of a vegetative cell into a heterocyst is repressed either by the presence of an external source of combined nitrogen such as ammonia or by the close proximity of neighboring heterocysts in the filament. In the absence of a combined nitrogen source, approximately 10% of the cells along a filament form heterocysts at a relatively constant interval, producing a simple one-dimensional pattern of development.Global changes in gene expression occur during heterocyst differentiation (5, 22). Most notably, nitrogen fixation (nif) genes are induced an...

Molecular characterization of planktic cyanobacteria of Anabaena, Aphanizomenon, Microcystis and Planktothrix genera.

“…Genomic DNA was extracted as described by Golden et al (1988). DNA from Aphanizomenon, Nodularia and Planktothrix strains was extracted by a slightly modified method, with the addition of sucrose (10 %, w\v) and lysozyme (10 mg ml − ") as described previously (Lehtima$ ki et al, 2000).…”

Characterization of Nodularia strains, cyanobacteria from brackish waters, by genotypic and phenotypic methods.

“…Genomic DNA was isolated from the cultures by a procedure modified from Golden et al (1988). Sucrose (20 % w\v), lysozyme (10 mg ml − "), sodium sarcosyl (1 % w\v) and proteinase K (200 µg ml − ") were used to lyse the cells.…”

Tansley Review No. 107. Heterocyst and akinete differentiation in cyanobacteria

“…During the late stages of heterocyst differentiation in Anabaena PCC 7120 three developmentally regulated DNA rearrangements occur in the genes nifD (encoding the α-subunit of dinitrogenase), fdxN (encoding a bacterial-type ferredoxin) and hupL (encoding the large subunit of an uptake hydrogenase) (Haselkorn, 1992 ;Tandeau de Marsac & Houmard, 1993 ;Apte & Prabhavathi, 1994 ;Matveyev et al, 1994 ;Apte & Nareshkumar, 1996 ;Bo$ hme, 1998 ;. These rearrangements involve the excision of an 11-kb element from nifD (Golden et al, 1988), a 55-kb element from fdxN (Golden et al, 1988), and a 10.5-kb element from hupL (Matveyev et al, 1994 ;Carrasco et al, 1995). These excisions are brought about by three excisases encoded by genes carried on the respective elements : xisA on the 11-kb element (Lammers et al, 1986) ; xisF on the 55-kb element (Carrasco et al, 1994) ; and xisC on the 10.5-kb element (Carrasco et al, 1995).…”