Background
The microbial transglutaminase (MTG) is inactive when only the mature sequence is expressed in
Pichia pastoris.
Although co-expression of MTG and its N-terminal pro-peptide can obtain the active MTG, the enzyme activity was still low. One of the basic steps for strain improvement is to ensure a sufficient level of transcription of the heterologous gene, based on promoter strength and gene copy number. To date, high-copy-number recombinants of
P. pastoris
are achievable only by cloning of gene concatemers, so methods for rapid and reliable multicopy strains are therefore desirable.
Results
The coexpression strains harboring different copies
mtg
were obtained successfully by stepwise increasing Zeocin concentration based on the rDNA sequence of
P. pastoris
. The genome of coexpression strains with the highest enzyme activity was analyzed by real-time fluorescence quantitative PCR, and three copies of
mtg
gene (
mtg-
3c) was calculated according to the standard curve of
gap
and
mtg
genes (
gap
is regarded as the single-copy reference gene). The maximum enzyme activity of
mtg-
3c was up to 1.41 U/mL after being inducted for 72 h in 1 L flask under optimal culture conditions, and two protein bands were observed at the expected molecular weights (40 kDa and 5 kDa) by Western blot. Furthermore, among the strains detected, compared with
mtg
-2c,
mtg
-6c or
mtg
-8c,
mtg
-3c is the highest expression level and enzyme activity, implying that
mtg
-3c is the most suitable for co-expression pro-peptide and MTG.
Conclusions
This study provides an effective strategy for improving the expression level of active MTG by directional increasing of
mtg
copies in
P. pastoris.
Electronic supplementary material
The online version of this article (10.1186/s12896-019-0542-6) contains supplementary material, which is available to authorized users.