Construction of DNA control plasmid designed for the 4 CDC Category A DNA agents (Smallpox virus [seq1], Bacillus anthracis [seq2], Francisella tularensis [seq3], and Yersinia pestis [seq4]). Assembling of the smallpox virus and B. anthracis sequences is presented as an example. Successive steps are indicated by framed numbers. 1, PCR amplification of the two matrix sequences by primers consisting of the stabilization and the restriction site sequences (italics). PCR reactions were carried out in a volume of 50 μl that included 10 mM Tris-HCl [pH 9.0], 1.5 mM MgCl2, 50 mM KCl, 0.1% Triton X-100, 200 μM each dNTP, 0.4 μM of each oligonucleotide primer, 0.4 μM of the single stranded DNA, and 1.5 U of Taq DNA polymerase (Invitrogen, Cergy-Pontoise, France). The thermocycler (Biometra, Göttingen, Germany) profile was 5 min at 95°C, followed by 35 cycles of 30 sec at 95°C, 30 sec at 55°C, and 1 min at 72°C, and terminated by a final extension for 7 min at 72°C. PCR products were electrophorezed in 3% TAE-agarose gel containing ethidium bromide and visualized under UV transillumination. Column purification of the PCR products. PCR products of the expected size were column-purified by using the QIAquick PCR Purification Kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions, and eluted in 50 μl of RNase free distillated water. When two bands or more were observed by gel analysis, the band of expected size was excised from the gel and purified by glass milk extraction with the GenClean III Kit (Q-Bio-Gene, Carlsbad CA, USA). 2, assemblage was conducted by pair, seq1 with seq2 (resulting in seq1-2). Equal volumes (10 μl) of purified seq1- and seq2-dsDNA were incubated at 37°C in the presence of Sac I. Sac I site is located at the 3' and 5' ends of seq1 and seq2, respectively. 3, the reaction product was column purified using the protocol aforementioned to discard the 15-nt DNA fragments corresponding to the 5' and 3' ends to avoid their re-ligation to their respective complementary sequences at step 5. 4, Overnight incubation at 4°C in the presence of T4 DNA ligase. Ten μl of the reaction was incubated with T4 DNA ligase (Roche, Basel, Switzerland) according to the manufaturer's instructions. 5, PCR amplification by using the external primers (italics) was performed according to the protocol described at step 1. Then column purification using the protocol detailed at step 2 of the resulting PCR product. At this step the seq1-2 PCR product may be cloned into PGEM-T for storage. The same procedure was performed for seq3 and seq4. Ultimately, seq1-2 and seq3-4 were assembled by using the same protocol (sections 1–9). The final product cloned into PGEM-T plasmid includes seq1-2-3-4 flanked by the two Sseq and restriction sites.