Genetic organization of some gene clusters identified during this study. a) Genomic region of the A. pleuropneumoniae 5b strain L20 genome surrounding ORFs encoding for genes PM0741 and NMB1668. ORFs ap2146 and ap2147 are located 260 pb downstream of the NMB1668 ORF, and are transcribed in the opposite direction. b) yfeAB and omp64 genes are separated by three ORFs that did not show differential expression. c) Genetic organization of a possible operon coding for a putative enterobactin-type ABC transporter system. The two ORFs separating fetB2 and NMB1993 are the putative cytoplasmic components of this hypothetical system. d) The Fe2+/Pb2+ high affinity permease locus.