Isolation, cloning, and expression of E. coli BirA gene for biotinylation applications

The key enzyme in biotin‑(strept) avidin systems, Escherichia coli BirA biotin ligase, is currently obtained by overexpression of the long protein‑tagged versions of the gene to prevent its toxic effect in E. coli. Herein we describe a rather simple and efficient system for expression of E. coli BirA without the application of long‑tag proteins.

The coding sequence of BirA gene was isolated by polymerase chain reaction using DNA extract of E. coli‑DH5α as template. BirA amplicon harboring a GS‑linker at its C‑terminal was cloned into NdeI‑XhoI sites of pET24a(+) vector under control of T7 promoter and upstream of the vector‑derived 6xHis‑tag. pET24‑BirA transformed BL21‑cells were induced for protein expression by IPTG and analyzed by SDS‑PAGE and Western blotting. Protein expression yields were assessed by image analysis of the SDS‑PAGE scans using ImageJ software.

Agarose gel electrophoresis indicated proper size of the BirA gene amplicon (963 bp) and accuracy of the recombinant pET24‑BirA construct. Sequence alignment analysis indicated identical sequence (100%) of our isolate with that of the standard E. coli‑K12 BirA gene sequence (accession number: NC_000913.3). SDS‑PAGE and Western blot results indicated specific expression of the 36.6 kDa protein corresponding to the BirA protein. Image analysis estimated a yield of 12% of total protein for the BirA expression.

By application of pET24a(+) we achieved relatively high expression of BirA in E. coli without application of any long protein‑tags. Introduction of the present expression system may provide more readily available source of BirA enzyme for (strept) avidin–biotin applications and studies.