Analysis of metal incorporation during overexpression of Clostridium pasteurianum rubredoxin by electrospray FTICR mass spectrometry

The rate of production of Clostridium pasteurianum rubredoxin overexpressed in Escherichia coli was examined by electrospray ionization-Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Previous work had shown that this heterologous expression resulted in isolation of both iron-containing (FeRd) and zinc-containing (ZnRd) rubredoxins. In the present work, minimally processed cell lysates of E. coli were analyzed in order to monitor the production of FeRd and ZnRd. The sensitivity of the measurement favored FeRd relative to ZnRd, and this differential sensitivity was quantitated using previously separated and purified rubredoxins. A time course study indicated that ZnRd and FeRd are produced simultaneously during overexpression, but at different rates. The ratio of the concentration of ZnRd to FeRd increased in a linear fashion during 3 h following induction of overexpression. Since only FeRds have been reported from native bacteria and archaea, the data suggest that either Zn2+ is sequestered from rubredoxins during native biosynthesis or that ZnRds may have escaped detection in the native microorganisms. ESI-FTICR mass spectrometry is shown to be a useful tool for monitoring metal insertion during protein biosynthesis.