Long stretches of sequential and identical serine or alanine codons are compatible with an efficient full-length protein expression in Escherichia coli

The Schistosoma japonicum glutathione S-transferase (GST) recombinant cDNAs, carrying blocks of sequential and identical triplets, consisting of 15-30-45 GCT (Ala) codons or 15-30 and also up to 75 AGC (Ser) codons, are expressed efficiently in an Escherichia coli system in the form of full-length protein chains, as detected by Coomassie-stained SDS-polyacrylamide gels, and soluble fusion proteins are purified by GSH-affinity chromatography. High expression levels and high yields of purified recombinant proteins are achieved. The efficient protein expression is independent of the molecular context and position of the polySer/polyAla string inserted into the GST carrier (near the part of the gene encoding the N- or the C-terminus). These findings suggest that E. coli is a powerful biological system to express foreign genes carrying long stretches coding for Ser- or Ala-rich domains, which are not uncommon in eukaryotic proteins. Moreover, data reported here show that the negative effect of sequential serine codons on protein expression in bacteria, previously reported in the literature, is not a general phenomenon.