An attempt is made to characterize the rapidly labeled hybridizable RNA of L5178Y @entity19 leukemic cells which has been shown to have similar base sequences when synthesized in two different stages of the cell cycle. The size of rapidly labeled RNA molecules was heterogeneous. For labeling times of 20 min or less, the per cent of hybridization was maximal. With longer labeling times, the per cent of hybridization decreased as radioactivity appeared in long-lived species of low hybridization efficiency; the radioactivity profile resembled the optical density profile in @entity969 gradients. The lifetime of newly synthesized hybridizable RNA was studied by pulse labeling exponentially growing cells and then "chasing" with nonradioactive @entity6683 . The per cent of hybridization was studied as a function of chase time. Three RNA groups, which comprised different proportions of rapidly labeled hybridizable RNA, were distinguished. The short-lived group had a half-life of 10 min, much less than the values reported in the literature for messenger RNA of @entity1 cells. The half-life of 1-1(1/2) hr observed for a medium-lived group more closely corresponds to that of messenger RNA. A long-lived group had a half-life of approximately 20 hr. Specific activity measurements during chase indicate the presence of a "pool" of labeled @entity6683 derivatives. The @entity6683 of this pool appears to be nonexchangeable with but dilutable by exogenous @entity6683 . A nontoxic concentration of @entity4316 was added to the chase media in an attempt to block the "pool effect". A rapidly degradable RNA was demonstrable both by specific activity and per cent of hybridization measurements.

Characterization of the rapidly labeled hybridizable RNA synthesized in L5178Y XXXX leukemic cells. Hybridization lifetime studies.

@entity1 :: ('9606', 'Species') :: ['mammalian'] @entity19 :: ('10090', 'Species') :: ['mouse'] @entity4316 :: ('MESH:D003609', 'Chemical') :: ['actinomycin D'] @entity969 :: ('MESH:D013395', 'Chemical') :: ['sucrose'] @entity6683 :: ('MESH:D014529', 'Chemical') :: ['uridine']
@entity19 :: (10090,Species) :: ['mouse']
