Automatic microinjection system facilitates detection of growth inhibitory mRNA

Naturally quiescent human lymphocytes, consisting predominantly of T cells, contain mRNA(s) that can inhibit DNA synthesis when injected into either human diploid fibroblasts (IMR-90) or transformed recipient cells (HeLa). By using an automated capillary microinjection system and a fluorescent coinjection marker (fluorescein isothiocyanate-dextran), individually injected cells can be retrieved and analyzed for DNA synthesis. mRNA isolated from resting T cells is able to block the cells from entering the S phase. The block is reversible and leads to a delay in DNA synthesis. The inhibitory effect is not observed if the injected mRNA is isolated from growth-activated T cells. The disappearance of the inhibition coincides with the approach of the G1/S boundary in both the donor T cells and the recipient human fibroblasts. The mRNA of resting T cells was size-fractionated and the peak inhibitory activity was recovered in a fraction approximately equal to 1.5 kilobases long.