Complete Genome Sequence of Bacillus subtilis Strain QB928, a Strain Widely Used in B. subtilis Genetic Studies

Abstract

The complete genome sequence of Bacillus subtilis strain QB928 was constructed to facilitate studies in the evolution of the genetic code. With a widespread use of the strain in Bacillus subtilis genetics studies, its complete genome sequence would facilitate deeper understanding of Bacillus subtilis genetics.

GENOME ANNOUNCEMENT

Bacillus subtilis is the workhorse of a wide range of industrial processes and the model organism for Gram-positive bacteria (4). The strain QB928 was previously constructed to aid in genetic mapping due to the presence of various markers [aroI(aroK)906 purE1 dal(alrA)1 trpC2] (3). Since then, it has been widely adopted in a wide range of bacterial studies. According to the Google Scholar search engine, QB928 has been cited by 171 articles, with topics ranging from genetic code modification through sporulation and industrial food production to antibiotic production. Most importantly, QB928 was used to generate a set of mutants leading to codon displacement phenotypes (7, 11); we set out to complete the genome for QB928 to facilitate downstream comparative genomic studies.

High-throughput DNA sequencing of QB928 was done on the Illumina GA IIx 75-bp paired-end platform with an average insert size of 200 bp at BGI-Shenzhen (BGI). Altogether, 4,954,514 reads were achieved, resulting in ∼89× coverage of the 4.15-Mbp QB928 genome. Reads were filtered to remove adapter sequences, low-quality bases (Phred score, <10), and singletons. A draft genome was generated by de novo assembly using Velvet 1.0.09 (12). Twenty-five scaffolds with an N₅₀ value of 488,188 were constructed. Gaps within scaffolds were iteratively closed using a previously described method (10). Gaps between scaffolds and the remaining gaps within scaffolds were closed by Sanger sequencing of the PCR products spanning gap regions.

The size of the QB928 genome is 4,146,839 bp, which is about 69 kbp smaller than the previously published Bacillus subtilis strain 168 genome (1). The mean GC content of QB928 is 43.61%. By the use of RATT (8), confidently homologous annotations from B. subtilis 168 were transferred to the QB928 genome. The ISGA server (5) was used for novel gene discovery. Altogether, 4,292 genes were annotated, composed of 4,113 coding genes, 30 rRNA genes in 10 rRNA operons, 86 tRNAs, and 63 miscellaneous RNAs (e.g., small RNAs). Whole-genome comparison using progressiveMauve (2) revealed 1,528 variants in QB928 with respect to B. subtilis 168. Nonsense mutations W115Stop and E175Stop were found in aroI (aroK) and dal (alrA), respectively, which are consistent with its tryptophan and d-alanine auxotrophic phenotype. However, no mutation can be found in purE. Instead, we found a missense E195K mutation in purC, which suggested that the genotype notation from the Bacillus Genetic Stock Center could be incorrect.

We found two large deleted regions in QB928 with respect to B. subtilis 168. The first region spans from positions 529424 to 549937 relative to B. subtilis 168, which contain genes found in or associated with transposons, or bacteriophages. The second region spans from positions 2653333 to 2701362 relative to B. subtilis 168, which contain the skin (sigK intervening) element (6) between spoIVCB and spoIIIC, and is assumed to be a remnant of a phage (9). In previous studies, it was shown that deletion of the skin element from B. subtilis would not impair growth or sporulation (6).

QB928 is one of the widely used Bacillus subtilis strains and is currently available from the Bacillus Genetic Stock Center. Its complete genome sequence should benefit the research community in general.

Nucleotide sequence accession number.

The sequence is accessible from GenBank under the accession number CP003783.