Exonuclease resistant 18S and 25S ribosomal RNA components in yeast are possibly newly transcribed by RNA polymerase II

RNAP II produces capped 18S and 25S ribosomal RNAs resistant to 5′-monophosphate dependent processive 5′ to 3′ exonuclease in polymerase switched Saccharomyces cerevisiae

Background

We have previously found that, in the pathogenic yeast Candida albicans, 18S and 25S ribosomal RNA components, containing more than one phosphate on their 5′-end were resistant to 5′-monophosphate requiring 5′ → 3″ exonuclease. Several lines of evidence pointed to RNAP II as the enzyme producing them.

Results

We now show the production of such 18S and 25S rRNAs in Saccharomyces cerevisiae that have been permanently switched to RNAP II (due to deletion of part of RNAP I upstream activator alone, or in combination with deletion of one component of RNAP I itself). They contain more than one phosphate at their 5′-end and an anti-cap specific antibody binds to them indicating capping of these molecules. These molecules are found in RNA isolated from nuclei, therefore are unlikely to have been modified in the cytoplasm.

Conclusions

Our data confirm the existence of such molecules and firmly establish RNAP II playing a role in their production. The fact that we see these molecules in wild type Saccharomyces cerevisiae indicates that they are not only a result of mutations but are part of the cells physiology. This adds another way RNAP II is involved in ribosome production in addition to their role in the production of ribosome associated proteins.

Identification of factors involved in ribosome assembly in the protozoan parasite Leishmania major

Formation of the ribosome subunits is a complex and progressive cellular process that requires a plethora of non-ribosomal transient proteins and diverse small nucleolar RNAs, which are involved from the synthesis of the precursor ribosomal RNA in the nucleolus to the final ribosome processing steps in the cytoplasm. Employing PTP-tagged Nop56 as a fishing bait to capture pre-ribosomal particles by tandem affinity purifications, mass spectrometry assays and a robust in silico analysis, here we describe tens of ribosome assembly factors involved in the synthesis of both ribosomal subunits in the human pathogen Leishmania major, where the knowledge about ribosomal biogenesis is scarce. We identified a large number of proteins that participate in most stages of ribosome biogenesis in yeast and mammals. Among them, we found several putative orthologs of factors not previously identified in L. major, such as t-Utp4, t-Utp5, Rrp7, Nop9 and Nop15. Even more interesting is the fact that we identified several novel candidates that could participate in the assembly of the atypical 60S subunit in L. major, which contains eight different rRNA species. As these proteins do not seem to have a human counterpart, they have potential as targets for novel anti-leishmanial drugs. Also, numerous proteins whose function is not apparently linked to ribosome assembly were copurified, suggesting that the L. major nucleolus is a multifunctional nuclear body.

Determinants of the temperature adaptation of mRNA degradation

The rate of chemical reactions increases proportionally with temperature, but the interplay of biochemical reactions permits deviations from this relation and adaptation. The degradation of individual mRNAs in yeast increased to varying degrees with temperature. We examined how these variations are influenced by the translation and codon composition of mRNAs. We developed a method that revealed the existence of a neutral half-life above which mRNAs are stabilized by translation but below which they are destabilized. The proportion of these two mRNA subpopulations remained relatively constant under different conditions, even with slow cell growth due to nutrient limitation, but heat shock reduced the proportion of translationally stabilized mRNAs. At the same time, the degradation of these mRNAs was partially temperature-compensated through Upf1, the mediator of nonsense-mediated decay. Compensation was also promoted by some asparagine and serine codons, whereas tyrosine codons promote temperature sensitization. These codons play an important role in the degradation of mRNAs encoding key cell membrane and cell wall proteins, which promote cell integrity.