Nutritional and Growth Control of Ribosomal Protein mRNA and rRNA in Neurospora crassa

Abstract

The effects of changing growth rates on the levels of 40S pre-rRNA and two r-protein mRNAs were examined to gain insight into the coordinate transcriptional regulation of ribosomal genes in the ascomycete fungus Neurospora crassa. Growth rates were varied either by altering carbon nutritional conditions, or by subjecting the isolates to inositol-limiting conditions. During carbon up- or down-shifts, r-protein mRNA levels were stoichiometrically coordinated. Changes in 40S pre-rRNA levels paralleled those of the r-protein mRNAs but in a non-stoichiometric manner. Comparison of crp-2 mRNA levels with those of a crp-2::qa-2 fusion gene indicated no major effect from changes in crp-2 mRNA stability. Crp-2 promoter mutagenesis experiments revealed that two elements of the crp-2 promoter, -95 to -83 bp (Dde box) and -74 to -66 bp (CG repeat) important for transcription under constant growth conditions, are also critical for transcriptional regulation by a carbon source. Ribosomal protein mRNA and rRNA levels were unaffected by changes in growth rates when the cultures were grown under inositol-limiting conditions, suggesting that, under these conditions, transcription of the ribosomal genes in N.crassa was regulated independently of growth rate.

Carbon Regulation of Ribosomal Genes in Neurospora crassa Occurs by a Mechanism Which Does Not Require Cre-1, the Homologue of the Aspergillus Carbon Catabolite Repressor, CreA

“…The levels of 40S rRNA and the mRNA levels of crp-1, crp-2, crp-3, crp-4, and crp2::qa2 are coordinated during carbon upshifts and downshifts (Shi and Tyler, 1991;Cujec and Tyler, 1996;de la Serna et al, submitted). A region which is important for regulation by carbon source contains two elements, the Dde box (Ϫ95 to Ϫ83) and the CG repeat (Ϫ73 to Ϫ66) (Cujec and Tyler, 1996b). Figure 8 shows that neither extensive substitution nor deletion :qa-2 mRNA levels during steady state at 25°C for 16 h in sucrose medium as described under Materials and Methods.…”

“…The levels of 40S rRNA increase by a factor of 7 while the mRNA levels of r-protein genes increase 1.5-to 2-fold during the first 2 h following an upshift from glycerol to sucrose (Cujec and Tyler, 1996b). Conversely, 40S rRNA and r-protein mRNA levels decrease to 10% of preshift levels during the first 2 h following a carbon downshift from sucrose to glycerol (Cujec and Tyler, 1996b). Furthermore, r-protein mRNA levels are coordinately expressed during steady state growth conditions.…”

“…Similarly, the levels of 40S rRNA and three r-protein mRNAs rapidly decreased to 10-20% of preshift levels within the first 2 h of a shift from sucrose to glycerol (Cujec and Tyler, 1996b). There was a rapid rise in these rRNA and r-protein mRNA levels during the first 2 h of an upshift from glycerol to sucrose (Cujec and Tyler, 1996b). These result suggest that transcriptional control is important in coordinating the expression of these ribosomal genes.…”

Non-coordinate regulation of 5S rRNA genes and the gene encoding the 5S rRNA-binding ribosomal protein homolog in Neurospora crassa

“…In N. Crassa the mRNA levels produced by four rprotein genes decrease coordinately during a carbon downshift from sucrose to quinic acid (Shi and Tyler 1991b) or to glycerol (Cujec and Tyler 1996b), and rise coordinately during an upshift from glycerol to sucrose (Cujec and Tyler 1996b). The levels of 40S precursor rRNA are also coordinated with the levels of mRNA for the r-protein genes (Cujec and Tyler 1996b).…”

“…The levels of 40S precursor rRNA are also coordinated with the levels of mRNA for the r-protein genes (Cujec and Tyler 1996b). The promoters of these genes have conserved sequences which could potentially coordinate their expression (Cujec and Tyler 1996a).…”