Using cloned lines of Plasmodium berghei producing mixed asexual and sexual (clone 234L) and purely asexual (clone 233L) parasitaemias, the courses of parasitaemia, gametocytogenesis, exflagellation, ookinete production in vitro and mosquito infectivity have been followed. For clone 234L mosquito infectivity is maximal at day 3 and has ceased by day 6 post-infection. Conversely, gametocytogenesis, exflagellation and ookinete production are at minimal levels at day 3 and rise to peaks between days 10 and 15 of infection (in TO mice infected with blood at mechanical blood passage 3). Sexual potential declines progressively with sustained mechanical passage (up to P14). Gametocyte conversion is highest early in infection and declines exponentially; however, upon each mechanical passage conversion is again raised but decreases more rapidly with succeeding passages. For practical mosquito transmission in the laboratory we consider P8 to be the useful limit for mechanical transmission for this parasite clone. Asexual parasite growth (virulence) is more rapid with increased mechanical passage inducing a rapid fall in haematocrit. By P14 the course of infection closely parallels that of the purely asexual clone 233L.
Rapidly labelled, polyadenylated RNA is contained in three distinct fractions isolated from homogenized amphibian oocytes: (a) in ribonucleoprotein particles that are associated with a fibrillar matrix, the complexes sedimenting at > 1500s; (b) in ribonucleoprotein particles that sediment at 20 -120s and have the characteristics of stored (maternal) messenger ribonucleoprotein (mRNP) and (c) in polyribosomes that sediment at 120 ~ 360 S. We have compared the RNA and protein components of the first two of these RNP fractions. The polydenylated RNA extracted from the two RNP fractions differs in that the RNA from fibril-associated RNP contains a much higher content of repeat sequences than does the RNA from mRNP. In other words, the RNA from fibrilassociated RNP is largely unprocessed and may constitute a premessenger state, which for convenience is referred to as premessenger RNP (pre-mRNP). RNA-binding experiments demonstrate that the polypeptide most tightly bound in pre-mRNP is a 54-kDa component (p54), whereas the polypeptide most tightly bound in mRNP is a 60-kDa component (p60). Antibodies raised against p60 are used to show that this polypeptide is a common major component of pre-mRNP and mRNP and that it is also located in oocyte nuclei. However the state of p60 is modified between the premessenger and stored message levels: the polypeptide in mRNP is heavily phosphorylated whereas the equivalent polypeptide in pre-mRNP is completely unphosphorylated. The relative roles of the presence of repeat sequences and phosphorylation of mRNA-associated protein in blocking translation are discussed.Both messenger RNA molecules and premessenger RNA molecules of amphibian oocytes are associated with classes of oocyte-specific proteins [l, 21. That oocytes should have categories of ribonucleoprotein (RNP) distinct from somatic cells probably relates to the special function of oocytes in accumulating mRNA throughout oogenesis. Most of the stored (maternal) mRNA is not utilized until egg maturation and early embryogenesis [3] and is somehow stabilized and prevented from being translated in oocytes. It is of general interest to understand how the block to translation occurs for it may represent a situation that arises, albeit less frequently, in other cell types.Recently it has been demonstrated that a mixture of oocyte mRNP proteins, when combined with globin mRNA in vitro to form reconstituted RNP particles, is effective in blocking its translation [4]. How this finding relates to the endogenous stored mRNA of oocytes is uncertain for, unlike mature globin mRNA, much of the stored mRNA of Xenopus (and sea urchin) oocytes is unprocessed : most molecules contain sequences transcribed from short repetitive DNA elements of the genome [ 5 , 61. The relevance of repeat sequences, possibly lying adjacent to coding sequences, to the block in translation of stored mRNA has recently been assessed. Up to 70% of the mass of polyadenylated RNA in Xenopus oocytes contains repeat sequences [7] and has been shown to be untranslatabl...
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