EXPRESSION OF PROTEINS ENCODED BY THE ESCHERICHIA-COLI CYN OPERON - CARBON DIOXIDE-ENHANCED DEGRADATION OF CARBONIC-ANHYDRASE

Cyanase catalyzes the reaction of cyanate with bicarbonate to give 2CO (2). The cynS gene encoding cyanase, together with the cynT gene for c arbonic anhydrase, is part of the cyn operon, the expression of which is induced in Escherichia coli by cyanate. The physiological role of c arbonic anhydrase is to prevent depletion of cellular bicarbonate duri ng cyanate decomposition due to loss of CO2 (M. B. Guilloton, A. F. La mblin, E. I. Kozliak, M. Gerami-Nejad, C. Tu, D. Silverman, P. M. Ande rson, and J. A. Fuchs. J. Bacteriol. 175:1443-1451, 1993). A Delta cyn T mutant strain was extremely sensitive to inhibition of growth by cya nate and did not catalyze decomposition of cyanate (even though an act ive cyanase was expressed) when grown at a low pCO(2) (in air) but had a Cyn(+) phenotype at a high pCO(2). Here the expression of these two enzymes in this unusual system for cyanate degradation was characteri zed in more detail. Both enzymes were found to be located in the cytos ol and to be present at approximately equal levels in the presence of cyanate. A Delta cynT mutant strain could be complemented with high le vels of expressed human carbonic anhydrase II; however, the mutant def ect was not completely abolished, perhaps because the E. coli carbonic anhydrase is significantly less susceptible to inhibition by cyanate than mammalian carbonic anhydrases. The induced E. coli carbonic anhyd rase appears to be particularly adapted to its function in cyanate deg radation. Active cyanase remained in cells grown in the presence of ei ther low or high pCO(2) after the inducer cyanate was depleted; in con trast, carbonic anhydrase protein was degraded very rapidly (minutes) at a high pCO(2) but much more slowly (hours) at a low pCO(2). A physi ological significance of these observations is suggested by the observ ation that expression of carbonic anhydrase at a high pCO, decreased t he growth rate.