Genome rearrangements duringAnabaenaheterocyst differentiation

摘要

The filamentous cyanobacteriumAnabaenasp. PCC 7120 produces specialized, terminally differentiated cells called heterocysts that are the sites of nitrogen fixation. The genome ofAnabaenaundergoes at least two specific developmentally regulated DNA rearrangements during heterocyst differentiation. One rearrangement involves the nitrogen-fixation genesnifH,nifD, andnifK. During heterocyst differentiation, an 11-kilobase (kb) DNA element is excised from the 3prime; coding region of thenifDgene by site-specific recombination within 11 base pair, directly repeated sequences present at the ends of the element. The excision results in formation of a completenifDcoding sequence and allows transcription of all three genes from a single promoter upstream ofnifH. The genexisA, located at one end of the 11-kb element, is believed to encode the site-specific recombinase responsible for excision of the element from thenifDgene. A second DNA rearrangement occurs close to the 5prime; end of thenifSgene and results in placing the genesrbcLandrbcSapproximately 10ensp;kb from thenifgene cluster. This rearrangement is also a deletion from the chromosome but involves a significantly larger segment of the genome, 55ensp;kb. The genomic breakpoints of thenifSrearrangement have been cloned and sequenced. The recombination sites show no homology to those involved in thenifDrearrangement. This suggests that each rearrangement is catalyzed by a different site-specific recombination system and that the two arrangements may be independently regulated.