Optical mapping as a routine tool for bacterial genome sequence finishing

Whole genome restriction maps of the sequenced strains were produced through optical mapping technology. These maps allowed rapid resolution of sequence assembly problems, permitted closing of the genome, and allowed correction of a large inversion in a genome assembly that we had considered finished.Our experience suggests that routine use of optical mapping in bacterial genome sequence finishing is warranted. When combined with data produced through 454 sequencing, an optical map can rapidly and inexpensively generate an ordered and oriented set of contigs to produce a nearly complete genome sequence assembly.Xenorhabdus species are symbiotic bacteria associated with insectivorous nematodes of the genus Steinernema (for review see [1]) They reside in a specialized segment of the nematode gut [2,3], and provide insecticidal proteins [4,5] and small molecules [6-10] that help to kill the insect larvae that are the prey of the nematode. Both organisms reproduce in the dead larvae, the Xenorhabdus colonize the young nematodes, and the cycle repeats [11]. Xenorhabdus are closely related to the enteric gamma proteobacteria such as Escherichia coli [12], and are an emerging model for both mutualism and pathogenicity in invertebrate hosts. To better understand the genetic basis of these relationships, we are sequencing the genomes of two Xenorhabdus species: X. nematophila ATCC 19061 and an X. bovienii strain from Monsanto's collection.In the course of this work, we found that the X. nematophila genome contained large numbers of highly repetitive DNA regions, and efforts to finish the genome stalled. We sought a means to produce whole-genome maps for comparison with the genomic DNA sequence, and identified optical mapping as a useful means to align and orient the genome sections in silico. In addition, we produced an optical map of a second genome that we had considered finished, and identified a large sequence inversion that would have otherwise been unnoticed.Eight-fold