Codon binding and translational properties of an isoaccepting lysine tRNA peculiar to virus-transformed Cells

Lysine tRNA and cell division: a G1 cell cycle mutant is temperature sensitive for the modification of tRNA5Lys to tRNA4Lys

Ts-694 is a temperature sensitive mutant of hamster cells which is blocked in the G1 phase of the cell cycle at the restrictive temperature of 39 degrees. A comparison of the Lys-tRNA isoacceptors by RPC-5 chromatography showed a decrease in tRNA5Lys and an increase in tRNA4Lys at 39 degrees. This was identical to the changes seen in confluent cultures at the permissive temperature of 33 degrees. These Lys-tRNA changes were not seen in ts-694 cells blocked in G1 by isoleucine deficiency, nor in two other G1 ts mutants at the restrictive temperature. Cells trapped in S phase by a thymidine block also contained decreased levels of tRNA4Lys when raised to 39 degrees. Both tRNA4Lys levels and cell division increased when the cells were returned to the permissive temperature. An in vitro assay was established for the modification of tRNA5Lys to tRNA4Lys with tRNA6Lys and tRNA2Lys as intermediates. The first reaction is the synthesis of tRNA6Lys which involves the introduction of a modified uridine at the third position of the anticodon. Extracts of 694 cells grown at 33 degrees were able to modify rat liver [3H] tRNA5Lys to tRNA6Lys and tRNA4Lys in vitro when assayed at 25 degrees but not at 39 degrees. Extracts of Balb/c 3T3 cells, however, were more active at 39 degrees than at 25 degrees showing that the normal enzyme is not temperature sensitive. Ts-694 cell tRNA, isolated from cells grown at 33 degrees was aminoacylated at both 25 degrees and 39 degrees with rat liver synthetases. tRNA4Lys was present at both temperatures indicating that ts-694 cells do not contain a temperature sensitive tRNA4Lys.

Competence and progression growth factors stimulate different tRNAlys modification reactions in BALB/C 3T3 cells

Sparse cultures of Balb/C 3T3 cells were growth arrested in a medium containing 1% calf plasma. This treatment caused a decrease in tRNA4lys, a tRNA species related to cell division, and a corresponding increase in tRNA5lys. A restoration of tRNA4lys levels and an increase in cell number was obtained by the addition of either 10% calf serum or a combination of fibroblast growth factor and melanocyte stimulating activity. Neither growth factor alone evoked a proliferative response. The progression factors, insulin and multiplication stimulating activity stimulated the rapid conversion of tRNA5lys to tRNA2lys, whereas the competence factors, fibroblast growth factor and platelet-derived growth factor stimulated a slight conversion of tRNA2lys to tRNA4lys.