Taq I methylase gene was cloned and expressed constitutively under the control of the tetracycline resistance gene promoter. The effect of Taq I methylase protection on the growth kinetics, plasmid stability and production of MBP-Taq I fusion protein by recombinant E. coli cells was investigated both in shake flasks and under controlled bioreactor conditions. Shake flask experiments indicated that the use of nutritionally richer medium formulations may improve the growth characteristics, plasmid stability and also the total and extracellular Taq I endonuclease recovery yields by the methylase protected cells. Under controlled bioreactor conditions, co-expression of the methylase gene led to higher plasmid stabilities and improved the excretion levels considerably.