Cloning and study of the genetic organization of the exe gene cluster of Aeromonas salmonicida.

The Aeromonas salmonicida (As) exe gene cluster, an additional member of the pul-related operon family required for general signal-sequence-dependent secretion of proteins from Gram- bacteria, was cloned in the broad-host-range cosmid pLAFR3. Twelve genes, exeC-N, were identified by partial nucleotide (nt) sequence analyses (exeE-N) or determination of the complete sequence (exeC and exeD). The organisation of the exeC-N genes is similar to that of several other operons of this family. These genes are arranged contiguously and are apparently transcribed in the same direction. On alignment of As and A. hydrophila exe sequences a 73-bp 'silent' deletion was identified close to the end of the As exeF gene. No gene encoding prepilin peptidase (the PulO homolog) was detected in this region. The exeN gene is evidently the last gene of this operon; it is followed by an ORF encoding a putative transcription regulator.