THE NATURE OF THE AMINO ACID POOL USED FOR PROTEIN SYNTHESIS IN RAT BRAIN SLICES

The incorporation into brain slice protein of externally provided [1-¹⁴C]valine was measured at varying levels of valine in the medium, under conditions of constant protein synthesis and equilibration of intracellular valine specific activity. The results indicate that the valine pool used for protein synthesis is not identical to the pool of total free valine. Neither does the incorporation solely occur from an extracellular pool which is in equilibrium with the incubation medium. The data are compatible with a two-site activation model in which aminoacylation of tRNA occurs at both an internal site utilizing amino acid from the intracellular pool and an external (possibly membranous) site converting extracellular valine directly to valyl-tRNA. A good fit to the experimental observations is also provided by a compartmented intracellular valine pool model.     

乙酸积累对基因工程菌培养的影响及与培养基pH的关系

在rIL-2工程菌K_(802)(pLY—4)高密度培养中,发现培养液中有大量代谢副产物乙酸积累,乙酸的存在对工程菌的生长和产物的表达均有明显的抑制作用,这种抑制作用是制约工程菌高密度培养的重要因素。为了减小这种抑制作用,初步研究了培养基pH与乙酸抑制作用的关系,发现适当提高培养基pH值,能减小乙酸的抑制作用;高密度培养时,提高培养基的pH后,虽然仍有大量乙酸积累,但产物的表达水平和菌密度都有提高。     

THE DETERMINATION OF ESCHERICHIA COLI I IN SEA WATER

SUMMARY: For the determination of Escherichia coli I in sea water lactose broth frequently gave higher presumptive and confirmed counts than MacConkey's broth. In the presumptive count there were 53 cases where lactose broth gave larger numbers than MacConkey's broth, with 11 equal counts, and only 25 cases with smaller counts ( P =0·00137). After confirmation the corresponding numbers of cases were 39, 10 and 27 ( P =0·134).
In the samples giving most probable numbers (MPN) of less than 100 E. coli /100 ml lactose broth was superior to MacConkey's broth ( P =0·021). At higher MPN values both media were satisfactory, but with highly polluted water MacConkey's broth might give better recoveries due to the suppression of high concentrations of non-coli-aerogenes bacteria.
Samples stored for 24 or 48 hr before testing gave higher presumptive recoveries when examined with lactose broth than with MacConkey's broth, the values for P being 0·028 and 0·0027 respectively.
It appears that lactose broth without inhibitory ingredients could be used with advantage in the examination of sea water.     

ENERGY OF MAINTENANCE IN ESCHERICHIA COLI.

McGrew, Sarah B. (Pennsylvania State University, University Park) and M. F. Mallette. Energy of maintenance in Escherichia coli. J. Bacteriol. 83:844-850. 1962.-Relatively dense populations of Escherichia coli B in log phase were used to detect utilization of exogenous glucose for maintenance without growth. Turbidity at 400 mmu was used as the measure of growth, since it should reflect changes in either cell number or size. A threshold level of glucose was observed below which turbidity did not change during short-time experiments. Repeated additions of glucose during prolonged incubation at 37 C either increased the turbidity slowly or maintained it, depending on the amount of glucose. Plate counts to follow viability showed slow decreases for 10 days, while the unfed controls lost viability quite rapidly. From these results it was concluded that E. coli specifically utilized exogenous glucose for maintenance, without growth. The conflict of this opinion with that of earlier workers is discussed and some implications suggested.     

STUDIES ON THE LACTASE OF ESCHERICHIA COLI

A "lactase solution" was prepared from Escherichia coli. The mechanism of its action has been studied and changes in the rate of hydrolysis under various conditions investigated. The hydrolysis of lactose by the enzyme approximates the course of reaction of the integrated Michaelis-Menten equation. One molecule of enzyme combines with one molecule of substrate. E. coli lactase is readily inactivated at pH 5.0, and its optimal activity at 36°C. is reached between pH 7.0 and pH 7.5. The optimal temperature for its action was found to be 46°C. when determinations were carried out after an incubation period of 30 minutes. Its inactivation by heat follows the course of a first order reaction, and the critical thermal increment between the temperatures of 45°C. and 53°C. was calculated to be 56,400 calories per mol. The enzyme is activated by potassium cyanide, sodium sulfide, and cysteine, and irreversibly inactivated by mercuric chloride, silver nitrate, and iodine. After inactivation with copper sulfate partial reactivation is possible, while the slight inhibition brought about by hydrogen peroxide is completely reversible. The possible structure of the active groups of E. coli lactase as compared with other enzymes has been discussed.     

灵芝富碘实验中氨基酸含量的探讨

在灵芝TW液体深层富碘培养过程中,碘元素能显著提高灵芝菌丝体中氨基酸的含量,不富碘的灵芝菌丝体氨基酸含量为26.95％,在培养基中添加0．1％碘化钾而获得的富碘灵芝菌丝体及富碘锌锗硒灵芝菌丝体氨基酸含量分别为35．37％、35．86％,另外其氨基酸的组成也相对发生了显著的变化,因而推测碘元素可能是灵芝中某些氨基酸含成过程中的辅基或是其激活因子。