A Role for Methionine in Division of Chlorella Vulgaris

Abstract

A key metabolic role can be ascribed to methionine on the basis of many demonstrable biochemical functions, even though these reactions have not all been proven to exist within one organism. Methionine is a constituent of proteins, (although there are instances in which it is reported absent (3) ) it also is a source of methyl groups for a number of cellular components, undoubtedly through the intermediate, S-adenosylmethionine, the actual methyl donor (9). Of the many transmethylations that are known a few seem to be more directly linked than others to plant cell division. The incorporation of the methyl group into lignin (6, 7) and into pectin (21, 25, 26) indicates a need for methionine in cell wall formation. There is also evidence from work with animals that the methyl group can be used to a limited degree for purine and pyrimidine synthesis (16,17). In microorganisms such a use has not been found (13), although S-adenosylmethionine can act as methyl donor to several non-naturally occurring purines (23). In plants, transmethylation from methionine is known to occur during the synthesis of several plant alkaloids (4, 5, 8, 12, 33) which as a chemical class show many resemblances to the purines and pyrimidines, but a specific connection between these reactions and plant nucleic acid synthesis is unknown. Other sulfurcontaining metabolites, notably sulfhydryl compounds, are recognized as playing a significant, although speculative (34, 35) role in the division of cells in general; recently data have been presented which show sulfur to be necessary for cells of Chlorella ellipsoidea to divide (14, 15). Similar evidence for methionine, however, is sparse. The idea that methionine may be intimately associated with division, stems from the observation that selenomethionine, a structural, competitive analogue of methionine, will prevent cells of Chlorella vulgaris from dividing but will allow them to maintain other cellular activities that lead to growth into giant cells (31). Analysis of these giant cells has revealed that there are increases in the rate of respiration, dry weight, and protein, and that the newly formed proteins lack methionine but contain other sulfur amino acids derived from sulfate (32). The ability of selenomethionine to replace methionine in several biological and biochemical systems (10, 11, 18, 36) suggests that in the uncoupled C. zulgaris cell there exist processes that can operate despite the substitution. The selective uncoupling effect, however, strongly implies that methionine participates in a specific, irreplaceable manner in the division mechanism of the C. vulgaris cell. This report is one of a series designed to characterize more fully the relation of methionine to division. The experiments described here were performed to see if additional methionine analogues could uncouple growth from division and if the response to selenomethionine occurred in organisms other than C. vulgaris.