Enzymatic evidence for a simultaneous operation of two one-carbon assimilation pathways during growth of<i>Arthrobacter</i>P1 on choline

Abstract

Assimilation of C~ compounds by aerobic microorganisms may proceed via one of the following cyclic pathways: (I) the ribulose bisphosphate (RuBP) pathway of CO 2 fixation (Calvin cycle), (II) the ribulose monophosphate (RUMP) pathway of formaldehyde fixation, (III) the serine pathway and (IV) the dihydroxyacetone (DHA) pathway of formaldehyde fixation [1,2]. Generally only one pathway is involved in carbon assimilation from one-carbon compounds in a particular organism, although the two variants of the RuMP cycle (the KDPGand FBP-variant) may operate simultaneously in Methylococcus capsulatus and Pseudomonas methanica [3]. However, it has been reported that Methylococcus capsulatus (strain Bath) during growth on methane, in addition to assimilating formaldehyde via the RuMP pathway, has the ability to fix CO 2 via the Calvin cycle [4]. As judged from the low activities of the key enzymes of this cycle it is evident that the Calvin cycle plays only a minor role in the overall synthesis of cell constituents from C~ compounds in this organism. In this communication we wish to report enzymatic evidence for an exception to the general rule that in a certain organism not more than one C~ assimilation pathway operates as the major path of carbon leading to the formation of cell material. Our results suggest that the facultative methylotroph Arthrobacter P1 [5] requires both a serine pathway as well as a RuMP cycle of formaldehyde fixation as indispensable routes during the metabolism of choline.