The gene
tab3 contains the DNA sequence for 15 aminoacids (aa) of the V3 loop from six different isolates of HIV-1, joined by the sequence AGGGA. This gene was fused to a DNA fragment encoding the first 26 aa of the human interleukin 2 (hu-IL2) and cloned in an expression plasmid. The fusion protein (TAB4) was expressed at high levels in different
Escherichia coli
strains. The removal of the hu-IL2 fragment from the plasmid abolished the expression of the protein. Similar results were obtained for plasmids pTAB7 and pTAB7SE, containing two copies of tab3 with and without the hu-IL2 stabilising sequence, respectively. Plasmids pVB1 and pVB2, in which the gene tab3 was fused to the bacterial signal peptides OmpA and PelB, were also evaluated. Proteins VB1 and VB2 were expressed at low levels and were not translocated to the
E. coli periplasm. Our results indicate that the lack of expression of proteins without hu-IL2 was neither a consequence of the stalling of ribosomes at rare AGA codons, nor a product of a more complex secondary structure around the Shine-Delgarno region or the AUG triplet. The analysis of steady-state levels of specific transcripts showed that differential availability of functional messenger ribonucleic acids determined these differences in the expression of the multi-epitope polypeptides.
