<p><strong>Cloning. </strong>All expression vectors were prepared by standard cloning techniques, propagated in Escherichia coli DH5&alpha;, and verified by sequencing.</p>
<p><strong>Bacterial expression. </strong>Luria-Bertani medium containing ampicillin (100 &micro;g/ml) was inoculated with E. coli BL21(DE3) cells carrying the appropriate construct.</p>