Hyperproduction of chitinase influences crystal toxin synthesis and sporulation of Bacillus thuringiensis

Abstract

Bacillus thuringiensis HD-73 was transformed with the endochitinase gene chiA74 under the control of a strong promoter (pcytA) and a 5' mRNA stabilizing (STAB-SD) sequence (HD-73-pEBchiA74). Expression levels were compared with those observed from the wild type strain (HD-73) and the recombinant HD-73 strain expressing chiA74 under the control of its native promoter (HD-73-pEHchiA74). The chitinolytic activity of HD-73-pEBchiA74 was markedly elevated, being ~58- and 362-fold higher than, respectively, HD-73-pEHchiA74 and parental HD-73, representing the highest levels of chitinase expression in recombinant B. thuringiensis reported to date. Parasporal crystals measured under transmission electron microscopy showed that HD-73 produced crystals of 1.235 (+/-0.214) and 1.356 (+/-0.247) mum in length when the bacterium was grown in respectively, NBS and NBS with glucose. Otherwise, HD-73-pEBchiA74 synthesized crystals of 1.250 (+/-0.222) and 1.139 (+/-0.202) mum in length when cultivated in NBS and NBS with glucose, respectively, values that showed a diminution of ~10 and 20% compared with crystals produced by HD-73-pEHchiA74 grown under the same conditions. Comparison of viable spore counts per ml showed that HD-73-pEBchiA74 produced fewest viable spores (1.5 x 10(9), 1.3 x 10(9)), compared to HD-73-pEHchiA74 (4.9 x 10(9), 5.3 x 10(9)) and HD-73 (6.8 x 10(9), 8.8 x 10(9)) when grown in NBS and NBS supplemented with glucose, respectively. No change in cellular protease activity was observed despite the overproduction of the chitinase.