Occurrence of a complex of aminoacryl-tRNA synthetases in lactating rat mammary gland.

(1) Postmicrosomal particles were isolated from the cytosol of lactating rat mammary gland. These particles contained up to 40% of the cytosol protein, and about 40% of the total aminoacyl-tRNA synthetase activity of the cytosol. (2) Sucrose density gradient centrifugation of these postmicrosomal particles showed that the activity of seventeen synthetases was distributed between one or other of two major gradient regions, corresponding to values of 20--28 S and 5--7 S. Arginyl and cysteinyl tRNA synthetases showed peaks in both major regions. (3) A complex of eight aminoacyl-tRNA synthetases (those for lysine, leucine, isoleucine, methionine, glutamine, arginine, cysteine and valine) of molecular weight 3 - 10(6) was isolated from the postmicrosomal particles by gel filtration and chromatography on DEAE-cellulose.