Increased leukotoxin production: Characterization of 100 base pairs within the 530 base pair leukotoxin promoter region of Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans leukotoxin (LtxA) is a major virulence factor that kills leukocytes permitting it's escape from host immune surveillance. A. actinomycetemcomitans strains can produce high or low levels of toxin. Genetic differences reside in the so called JP2 ltxA promoter region. These hyper-leukotoxin producing strains with the 530 bp deletion have been studied in detail. However, regions contained within the 530 bp deletion that could be responsible for modulation of leukotoxin production have not been defined. Here, we report, for the first time, on regions within the 530 bp that are responsible for high-levels of ltxA expression. We constructed a deletion of 530 bps in a primate isolate of A. actinomycetemcomitans, which produced leukotoxin equivalent to the JP2 strain. We then constructed sequential deletions in regions that span the 530 bps. Results indicated that expression of the ltxA transcript was reduced by a potential transcriptional terminator in promoter region 298 to 397 with Delta G = -7.9 kcal/mol. We also confirmed previous findings that transcriptional fusion between the orfX region and ltxC increased ltxA expression. In conclusion, we constructed a hyper-leukotoxin producing A. actinomycetemcomitans strain and identified a terminator located in the promoter region extending from 298-397 that alters ltxA expression.