Molecular characterization of the tox operon involved in toxoflavin biosynthesis of Burkholderia glumae

Abstract

Two toxoflavin biosynthesis-related proteins (TRP-1, TRP-2) from wild strains of the phytopathogen Burkholderia glumae were previously identified, and toxA was determined to encode TRP-1, which has characteristics of a methyltransferase. An 8.2-kb region in the chromosomal DNA of B. glumae that contains the tox operon (toxABCDE) and an upstream regulatory gene (toxR) involved in phytotoxin toxoflavin biosynthesis was cloned and sequenced in this study. The sequence downstream of toxA contains four open reading frames -toxB, toxC, toxD and toxE -which encode polypeptides with calculated molecular masses of 23.3, 61.6, 34.9, and 38.3 kDa, respectively, all having the same transcriptional orientation as toxA. Mutants disrupted in the tox operon lost their ability to produce toxoflavin and did not induce typical chlorosis on infected rice panicles. Based on results from reverse transcription-polymerase chain reaction experiments, the message encoded by the tox operon may be polycistronic for all five genes. Also, the toxR gene was located upstream of this operon. The toxR gene product, with a calculated molecular mass of 37.5 kDa, might be a member of the LysR family of regulatory molecules and an activator that allows transcription of the tox operon. Furthermore, the deduced proteins of ToxB and ToxE had significant similarity to the GTP cyclohydrolase II and the deaminase, respectively, involved in riboflavin synthesis in several organisms. These results suggest that toxoflavin is synthesized in part through a biosynthetic pathway common to the synthesis of riboflavin, starting with GTP as the precursor.

Evolution of New Function in the GTP Cyclohydrolase II Proteins of Streptomyces coelicolor

“…One of the homologues is located just downstream of SCO 2687, and the role of this protein will be described below. Nevertheless, it is reasonable to posit that the cluster that bears SCO 1441 is involved in the biosynthesis of riboflavin.The biological role of the cluster bearing SCO 2687 has been tentatively assigned by comparison to a cluster of proteins in Burkholderia glumae responsible for formation of toxoflavin (30,31). In retrospect, this is perfectly reasonable and explains the presence of GCH II and the riboflavin-specific deaminase/reductase in this cluster.…”

Involvement of a Quorum-Sensing-Regulated Lipase Secreted by a Clinical Isolate of Burkholderia glumae in Severe Disease Symptoms in Rice

“…Burkholderia glumae is pathogenic to rice (Oryza sativa), producing symptoms that include seedling blight, seedling rot, and grain rot; all these symptoms are often considered manifestations of a single disease which is usually called panicle blight (11,22,23,37,41). Panicle blight is a recurring problem in rice-producing areas in the United States, Japan, and Korea, and its incidence has been increasing in recent years.…”

Amino Acid Substitutions in GyrA ofBurkholderia glumaeAre Implicated in Not Only Oxolinic Acid Resistance but Also Fitness on Rice Plants

“…Toxoflavin is a phytotoxin, produced by B. glumae, with a broad host range and is involved in the development of bacterial grain rot (35,39). The production of toxoflavin in B. glumae and its transportation into plant cells are regulated by quorum sensing through N-octanoyl homoserine lactone (18).…”