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Biomedical subjects

H L Cooper

Publications and source records attributed to H L Cooper.

At least 73 records · Page 4Linked to original sources
PubMed1978

The relationship between hnRNA+-poly(A) and mRNA+-poly (A) in non-dividing human lymphocytes. Evidence for distinct synthetic pathways for mRNA precursor- and nonprecursor-hnRNA.

The processing of hnRNA+-poly(A) to mRNA+-poly(A) has been studied in resting lymphocytes from human peripheral blood. In pulse-chase experiments, two types of hnRNA+-poly(A) have been distinguished: the first is labeled predominantly with exogenous radioactive precursors supplied during the pulse, and the second incorporates primarily scavenged labeled precursors made available during a chase incubation. When the disappearance of both types of hnRNA+-poly(A) was quantitatively compared with the appearance of stable and labile mRNA+-poly(A), only 10% of the anticipated cytoplasmic material was actually obtained. Statistically, 90% of the poly(A)-bearing hnRNA molecules processed were degraded. The two types of hnRNA+-poly(A) were found to be functionally different. Pulse-labeled material was processed to poly(A)-bearing mRNA; "chase-labeled" molecules did not leave the nucleus and never served as precursors for cytoplasmic mRNA. The data fit a model in which there are distinct pathways for mRNA precursor- and nonprecursor-hnRNA+-poly(A).

Cell Nucleus↗
PubMed1977

Free fibosomes and growth stimulation in human peripheral lymphocytes: activation of free ribisomes as an essential event in growth induction.

During the initial ten hours of growth in lymphocytes stimulated by phytohemagglutinin, the cells are converted from a state in which over 70% of all ribosomes are inactive free ribosomes, to one in which over 80% of ribosomes are in polysomes or in native ribosomal subunits. In this initial period, there was a neglible increase in total ribosomal RNA due to increased RNA synthesis, and abolition of ribosomal RNA synthesis with low concentrations of actinomycin D did not interfere with polysome formation. Therefore, the conversion is accomplished by the activation of existing free ribosomes rather than by accumulation of newly synthesized particles. The large free ribosome pool of resting lymphocytes is thus an essential source of components for accelerated protein synthesis early in lymphocyte activation, before increased synthesis can provide a sufficient number of new ribosomes. Free ribosomes accumulate once more after 24 to 48 hours of growth, when RNA and DNA synthetic activity are maximal. This reaccumulation of inactive ribosomes at the peak of growth activity may represent preparation for a return to the resting state where cells are again susceptible to stimulation. Activation of free ribosomes to form polysomes appears to involve modification of at least two steps: (a) dissociation of free ribosomes with stabilization as native subunits, and (b) adjustment of a rate-limiting step at initiation.

DNA↗
PubMed1976

Free ribosomes in physiologically nondividing cells. Human peripheral lymphocytes.

Ribosomes of physiologically nondividing human peripheral blood lymphocytes were studied by ultraviolet absorbance measurements of cytoplasmic extracts subjected to ultracentrifugation in sucrose gradients at high and low ionic strength. At least 70% of the total cytoplasmic ribosomes were free ribosomes which sedimented at 80 S in low salt and dissociated to 40 S and 60 S subunits in high salt. These particles were not involved in protein synthesis. The remaining ribosomes were equally divided among native subunits, active monosomes, and larger polysomes. Free ribosomes were shown to exist as 80 S particles in the intact cell, and labeling studies indicated that they did not freely return to the pools of protein-synthesizing components. New ribosomes appeared first as native subunits and in polysomes. After a lag of 15 to 20 min, the particles began to enter the free ribosome pool. Thus, free ribosomes arise in the resting cell by a unidirectional flow which continuously removes particles from the protein-synthesizing pool and sequesters them as an accumulation of inactive 80 S particles. The transition from native subunits to free ribosomes is accompanied by functional changes in association behavior of subunits and by alteration of sedimentation behavior of the subunits. These changes may be due to absence of a protein or proteins from the free ribosomes which is required to permit effective dissociation of subunits prior to initiation of translation. Deficiency of this dissociation factor may be responsible for the continuous formation of free ribosomes in resting cells. Our data also suggest a limitation of the rate of initiation of protein synthesis which may result from deficiency of an initiation factor.

Cell Division↗
PubMed1975

Very short-lived and stable mRNAs from resting human lymphocytes.

The kinetics of degradation of newly synthesized cytoplasmic poly(A)-bearing RNA have been examined in resting human lymphocytes. Two classes were identified, a very labile component with a half-life of less than 17 min and a stable component which remains apparently undiminished during 24 hr of observation. Both classes have molecular weights between 2.5 and 3.5 x 10(5) but the stable material has a narrower size distribution and a slightly lower average molecular weight than the short-lived component. The fate of stable RNA synthesized in the resting cell was also examined after growth stimulation with phytohemagglutinin after 2 and 6 hr of treatment. No transfer of stable material into the labile pool could be discerned; the amount of stable material remained constant. The existence of two species of mRNAs with different lifetimes in animal cells provides a potential means for regulation of protein synthesis by controlling the supply of specific messages. Furthermore, such a short-lived mRNA species may explain the observed disparity between the amount of poly(A)-bearing heterogeneous RNA produced in the nucleus and the amount of mature message found in the cytoplasm.

Cytoplasm↗