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The 7 kb virulence Region-2 of the large (virulence) plasmid in Shigella flexneri 2a encodes several proteins required for invasion of intestinal epithelial cells. Insertion and deletion mutagenesis, DNA subcloning and SDS-polyacrylamide gel electrophoresis of proteins synthesized in minicells demonstrated five genes in this region. They encode 24, 18, 62 (IpaB), 41 (IpaC) and 37 (IpaD)-kiloDalton (kD) proteins. Complementation of Tn5-induced mutations in Region-2 with the above plasmid constructs indicated that Region-2 consists of two operons and that the three Ipa proteins are essential for the virulence phenotype. The transcriptional organization determined by Northern blotting, S1 nuclease protection and the effect of Tn5 insertions on expression of the Ipa proteins revealed that Region-2 has three promoters that transcribe RNAs of 4.0, 4.5 and 7.5 kb. The 4.0 kb RNA was the transcript for the operon encoding the 24, 18 kD, IpaB and C proteins and the 4.5 kb RNA for the ipaD gene. In addition, the full-length RNA of 7.5 kb which covers Region-2 supplemented full expression of the Ipa proteins. The 7663 nucleotides of Region-2 were determined to confirm the five open reading frames encoding 23,655, 17,755, 62,168, 41,077 and 36,660 Dalton proteins, respectively, and their regulatory sequences.
Sasakawa et al. (Fri,) studied this question.