Carlos Morel é o autor vinculado à Fiocruz, mas não consta à informação no documento.Messenger RNA (mRNA) and messenger like RNA (mlRNA) were investigated in the cyto- plasm of HeLa cells while ribosomal RNA synthesis was arrested. Under these conditions, func- tional mRNA associated in polyribosomes and cytoplasmic free mlRNA are formed and can be labelled selectively to steady state. All cytoplasmic non-ribosomal RNA sedimenting at more than 6-7 S exists in the form of ribonucleoprotein complexes which pre-exist in the cell, and are stable upon cell lysis, sedimen- tation and (after fixation) CsCl density gradient analysis. The functional, true mRNA is contained in a complex of mRNA and protein which bands in association with ribosomes (e = 1.52 to 1.60 g/cm3) in CsCl density gradients or, released by EDTA, at its own intrinsic density of 1.40-1.88 g/cm3. The cytoplasmic free mlRNA bands as a particle of mlRNA-containing ribo- nucleoprotein at an identical low density. The molecular weight spectrum of mRNA is identical to that of mlRNA and the sedimentation pattern of the mRNA-protein complex released from polyribosomes is similar to that of the free mlRNA - protein complex. The physico-chemical separation of mRNA - and mlRNA - protein complexes allowed us to follow their relative kinetics of synthesis and decay. Each type of ribonucleoprotein obeys a different, strictly time-dependent pattern. Label enters the pool of free mlRNA - protein com- plexes first and may, in a pulse-chase experiment, be partially chased into polyribosomes. At steady state (6 h) 40-60°/, of the labelled RNA remains in the form of free mlRNA-protein particles. These cannot be chased into polyribosomes, the kinetics of mRNA. and mlRNA- protein complexes decay following identical patterns. These findings are in agreement with a model according to which mlRNA from the nucleus first joins the pool of free ribonucleoprotein. Then, the activated mRNA- protein complexes attach to ribosomal subunits and form polyribosomes whereas inactivated mlRNA * protein complexes remain free in the cytoplasm. In order to further strengthen the evidence in favour of the real existence of mRNA- and mlRNA * protein complexes in the cells, the corresponding fractions from sedimentation or CsCl density gradients were observed in the electron microscope. By this method it was possible to see small cytoplasmic particles which have not before been identified. The rounded structures, with diameters ranging from 100 h to 200 8, seem to consist of the coiling of a 35 8 wide pearl- like chain which may also be identsed in polyribosomes. The frequency of these particles is highest in the mlRNA. protein band (e = 1.40-1.48 g/cmS). Thus they may correspond to the mRNA - protein complex. However since they share some morphological features with other known biological structures the evidence is not conclusive.