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In addition to catabolic and anabolic pathways, which enable nutrients to enter and to be withdrawn from the central routes of metabolism, enzymic sequences operate to ensure that intermediates, removed from these central routes in the course of biosynthesis, are replenished. These obligatory path ways of replenishment are generically designafed "anaplerotic sequences". The nature of these sequences, operative in micro-organisms growing on C3-and C2-compounds, is discussed in outline, and the mode of regulation of one of them (the glyoxylate cycle) is examined in detail.[25] J. M . Ashworth and H . L. Kornberg, Biochim. biophysica Acta 89, 383 (1964).[26] P. Falmagne, E. Vanderwinkel, and J. M . Wiame, Arch. internat. Physiol. Biochim. 71, 813 (1963). A 4471228 IE] G e r m a n version: Angew. Chem. 77, 601 (1965) Angew. Chem. internut. EdiI. / Vol. 4 (1965) / No. 7

Escherichia coli inducibly form a system effecting the uptake of a number of C,-dicarboxylic acids; glucose or inhibitors of protein synthesis prevent this induction. After growth on C,-dicarboxylic acids, washed cells take up such acids rapidly and, initially, linearly with time; the K , for this process is approx. 30 pM and the V,,, is approx. 25 nmoles x min-1 x mg dry wt-l, irrespective of the identity of the acid transported. C,-dicarboxylic acids other than fumarate, succinate, malate, aspartate, and maleate, are taken up poorly. Washed cells taking up 14C-labelled C,-dicarboxylic acids incorporate isotope mainly into macromolecular cell components ; no significant intracellular accumulation of C,-acids is observed. Mutants blocked in component reactions of the tricarboxylic acid cycle are also greatly impaired in their ability to take up C,-acids; mutants blocked in reactions ancillary to the cycle are not. It is suggested that E . coli effect the uptake of C,-dicarboxylic acids through a common, inducible and highly specific transport system. The first step in the microbial utilization of any nutrient is its entry into the cells. It has long been recognized that this process is usually not one of passive diffusion, but involves the necessary functioning of specialized permeation systems, which may exhibit a high degree of stereospecificity towards the substrate transported [I]. Although much has been learned about the nature and control of the systems that effect the transport of uncharged molecules, such as hexoses [2] [11,12]. However, the nature and specificity of such processes remain to be resolved.It is the purpose of this paper to describe a system necessarily involved in the uptake of a number of C,-dicarboxylic acids by E . coli. MATERIALS AND .METHODSGrowth of Organisms Used All organisms used in this study were derivatives of E . coli, Ki2. Their characteristics are listed in Table 1. Cells were grown aerobically at 37" in a Gallenkamp incubator shaker, either in Oxoid nutrient broth or in defined media, containing salts 1151, necessary amino acids (at 50-100 pg/ml), and appropriate carbon sources at 25mM. Growth was measured as absorbance a t 680 nm; an absorbance of 1 corresponded to 0.68 mg dry wt/ml. Measurement of C,-Acid UptakeCells were harvested in the logarithmic phase of growth, washed with and resuspended in carbon-free salts medium [15]. The cell concentration was ad-