MEASUREMENT OF PROTEIN-SYNTHESIS BY SOIL BACTERIAL ASSEMBLAGES WITH THE LEUCINE INCORPORATION TECHNIQUE

The incorporation of [C-14]-leucine into protein by soil organisms was measured both in soil slurries and for bacteria extracted from soil b y homogenization - centrifugation. The result was compared with thymid ine incorporation. Using a soil slurry, 9.1 x 10(-10) mol leucine h-1 g-1 dry weight of soil was incorporated into protein, with a calculate d leucine: thymidine ratio (mol: mol) of about 34. Non-specific labell ing of macromolecules other than protein was observed with both the so il slurry and the homogenization-centrifugation method. With the latte r, 46.5% of the total incorporation was found in the protein fraction (hot-acid insoluble). The incorporation of leucine was linear with tim e for at least 4 h for extracted bacteria. Even at 2000 nM, [C-14] -le ucine did not saturate incorporation into protein. Isotope dilution pl ots indicated that with 750 nM leucine, the degree of participation of the labelled substance in protein synthesis was 0.59. With this value , the ratio of leucine: thymidine incorporation into total macromolecu les was calculated as 41 for extracted bacteria. On the basis of incor poration into protein (leucine) and incorporation into DNA (thymidine) only, the leucine: thymidine ratio was calculated as 117. The mean tu rnover time of bacteria at 22-degrees-C, calculated using conversion f actors from published studies and leucine incorporation into protein o f extracted bacteria, was 4.3 days.