Purification, crystallization and preliminary X-ray studies of two isoforms of Rubisco from Alcaligenes eutrophus

Phylogeny and functional expression of ribulose 1,5-bisphosphate carboxylase/oxygenase from the autotrophic ammonia-oxidizing bacterium Nitrosospira sp. isolate 40KI

The autotrophic ammonia-oxidizing bacteria (AOB), which play an important role in the global nitrogen cycle, assimilate CO(2) by using ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO). Here we describe the first detailed study of RubisCO (cbb) genes and proteins from the AOB. The cbbLS genes from Nitrosospira sp. isolate 40KI were cloned and sequenced. Partial sequences of the RubisCO large subunit (CbbL) from 13 other AOB belonging to the beta and gamma subgroups of the class Proteobacteria are also presented. All except one of the beta-subgroup AOB possessed a red-like type I RubisCO with high sequence similarity to the Ralstonia eutropha enzyme. All of these new red-like RubisCOs had a unique six-amino-acid insert in CbbL. Two of the AOB, Nitrosococcus halophilus Nc4 and Nitrosomonas europaea Nm50, had a green-like RubisCO. With one exception, the phylogeny of the AOB CbbL was very similar to that of the 16S rRNA gene. The presence of a green-like RubisCO in N. europaea was surprising, as all of the other beta-subgroup AOB had red-like RubisCOs. The green-like enzyme of N. europaea Nm50 was probably acquired by horizontal gene transfer. Functional expression of Nitrosospira sp. isolate 40KI RubisCO in the chemoautotrophic host R. eutropha was demonstrated. Use of an expression vector harboring the R. eutropha cbb control region allowed regulated expression of Nitrosospira sp. isolate 40KI RubisCO in an R. eutropha cbb deletion strain. The Nitrosospira RubisCO supported autotrophic growth of R. eutropha with a doubling time of 4.6 h. This expression system may allow further functional analysis of AOB cbb genes.

The crystal structure of rubisco from Alcaligenes eutrophus reveals a novel central eight-stranded beta-barrel formed by beta-strands from four subunits

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is involved in photosynthesis where it catalyzes the initial step in the fixation of carbon dioxide. The enzyme also catalyzes a competing oxygenation reaction leading to loss of fixed carbon dioxide, thus reducing the net efficiency of photosynthesis significantly. Rubisco has therefore been studied extensively, and a challenging goal is the engineering of a more photosynthetically efficient enzyme. Hexadecameric rubiscos fall in two distinct groups, "green-like" and "red-like". The ability to discriminate between CO2 and O2 as substrates varies significantly, and some algae have red-like rubisco with even higher specificity for CO2 than the plant enzyme. The structure of unactivated rubisco from Alcaligenes eutrophus has been determined to 2.7 A resolution by molecular replacement and refined to R and Rfree values of 26.6 and 32.2 %, respectively. The overall fold of the protein is very similar to the rubisco structures solved previously for green-like hexadecameric enzymes, except for the extended C-terminal domains of the small subunits which together form an eight-stranded beta-barrel which sits as a plug in the entrance to the central solvent channel in the molecule. The present structure is the first which has been solved for a red-like rubisco and is likely to represent a fold which is common for this group. The small subunits in general are believed to have a stabilizing effect, and the new quaternary structure in the oligomer of the present structure is likely to contribute even more to this stabilization of the assembled rubisco protein.