The effects of growth factors on tRNALys modification.

The tRNALys population from tissue culture cells contains several isoaccepting species which are not present in the tRNALys population from tissue sources. These isoacceptors were isolated from mouse LM cells and tested for their coding properties in ribosomal binding assays and their ability to incorporate lysine into protein in a reticulocyte lysate. tRNALys5A and tRNALys6 eluted in the area of tRNALys5. All three species coded preferentially for AAA and bound with equal efficiency. tRNALys1, tRNALys3, tRNALys4, and tRNALys6 all transferred lysine into protein at a slower rate than tRNALys2 and tRNALys5, which are the native species. Several purified growth factors were tested for their ability to affect the levels of tRNALys4, a tRNA possibly associated with cell division. When Balb/c 3T3 cells were grown in medium containing plasma instead of serum, there was a decrease in tRNALys2, tRNALys3, tRNALys4 and an increase in tRNALys5 and tRNALys6. The addition of either FGF or PDGF returned the tRNALys profile to normal. The extent of the tRNALys changes depended upon the concentration of growth factor added. FGF was able to cause a 35% decrease in the tRNALys5 peak with a corresponding increase in tRNALys2 within 1 h of the addition of the factor. These data suggest that competence factors have the ability to stimulate the modification of specific tRNALys isoacceptors.