Identification of in vivo induced genes in Actinobacillus pleuropneumoniae

We have developed an in vivo expression technology (IVET) system to identify Actinobacillus pleuropneumoniae gene promoters that are specifically induced in vivo during infection. This system is based upon an avirulent riboflavin-requiring A. pleuropneumoniae mutant and a promoter-trap vector (pTF86) that contains, in sequence, the T4 terminator, a unique Bam HI site, a promoterless copy of the V. harveyi luxAB genes, and a promoterless copy of the B. subtilis ribBAH genes in the E. coli - A. pleuropneumoniae shuttle vector pGZRS19. Sau 3A fragments of A. pleuropneumoniae genomic DNA were cloned into the Bam HI site in pTF86 and transformed into the A. pleuropneumoniae Rib- mutant. Pigs were infected with pools of 300-600 transformants by endobronchial inoculation and surviving bacteria were isolated from the pigs' lungs at 12-16 h post-infection. Infection strongly selected for transformants containing cloned promoters which drove expression of the vector ribBAH genes and allowed survival of the Rib- mutant in vivo. Strains that survived in vivo, but which minimally expressed luciferase activity in vitro, should contain cloned promoters that are specifically induced in vivo. Ten clones, designated iviA-J, were isolated which contain promoters that are induced in vivo during infection. These ivi clones were shown to be induced in the animal by luminescence of infected tissue and by direct assay of bacteria recovered from bronchoalveolar lavage. Four of these clones were putatively identified by amino acid sequence similarity as ilvI, the ilvDA operon, the secE-nusG operon, and the mrp gene. This is the first report of an IVET system for use in the family Pasteurellaceae, as well as the first report of an IVET system utilizing an infection model of pneumonia in the natural host.