Transcriptional analysis of the Aeromonas salmonicida S-layer protein gene vapA

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Abstract
The vapA gene of Aeromonas salmonicida encodes the subunit of the surface protein array known as A-layer. Nucleotide sequence analysis of the 374 bp of DNA immediately upstream of vapA revealed two potential promoter sequences and other possible regulatory sequences. Sequencing and polymerase chain reaction analysis showed that the region was conserved in wild-type A. salmonicida. Primer extension and Northern (RNA) blot analysis showed that vapA transcription in A. salmonicida was directed predominantly by a distal promoter, P1, resulting in a 1.7-kb unit-length mRNA with an untranslated 181-nucleotide leader sequence which contained two predicted low-free-energy stem-loop structures. Northern analysis of cells grown at 15 degrees C showed that vapA transcript production peaked during the mid-log phase of growth (A600 = 0.25). At 15 degrees C, the half-life of the vapA mRNA was 22 min, while at 20 degrees C, the half-life was significantly shorter, 11 min. The amount of vapA transcript produced was reduced by growth in the presence of the DNA gyrase inhibitors nalidixic acid and novobiocin. Environmental factors such as growth temperature and atmospheric oxygen tension also affected the quantity of vapA mRNA. vapA transcript could not be detected in mutants which produced either low levels of full-length or truncated A protein or no detectable A protein.