Significance of ribosomal ribonucleic acid synthesis for control of the G1 period in the cell cycle of the heterobasidiomycetous yeast Rhodosporidium toruloides

A transcriptional autoregulatory loop for KIN28-CCL1 and SRB10-SRB11, each encoding RNA polymerase II CTD kinase-cyclin pair, stimulates the meiotic development of S. cerevisiae

Alkalization of the medium is associated with and required for the cellular development to meiosis and sporulation in the yeast Saccharomyces cerevisiae. To elucidate the molecular mechanisms for the significance of external alkalization, we isolated mutants defective in division arrest at G1 phase under an alkaline condition. The mutants obtained had recessive alleles of SRB10 encoding the cyclin (SRB11)-dependent protein kinase that phosphorylates the CTD domain of the largest subunit of RNA polymerase II and negatively regulates the transcriptional initiation of certain genes. A delta srb11 deletion mutant showed the same cell cycle defect. When shifted to alkali, wild-type cells decreased transcript levels of G1-cyclin genes (CLN1 to CLN3) and KIN28-CCL1 (encoding another CTD kinase-cyclin pair which, in contrast, stimulates the promoter clearance and transcriptional elongation in most genes), resulting in the accumulation of G1 cells and the hypophosphorylated form of RNA polymerase II and in an increase in cell size. However, under the same conditions, a delta srb10 mutant was defective in these events, except the downregulation of CLN1 and CLN2. The delta srb10 mutation also influenced on the transcript levels of meiosis-inducing genes called IME1 and IME2: the mutation elevated the transcript level of IME1 but reduced that of IME2, resulting in partial defects in premeiotic DNA synthesis and meiosis. Overexpression of KIN28 and CCL1 in wild-type cells impaired the alkali-induced G1 arrest and the rate of meiosis and elevated the transcript levels of SRB11 and IME1. These results indicate that a transcriptional autoregulatory loop for KIN28-CCL1 and SRB10-SRB11 is important for G1 arrest and meiosis. We also found that environmental conditions for meiosis finely regulate the transcript levels of KIN28 and CCL1, such that nitrogen starvation first elevates them but subsequent alkalization of medium decreases them.

Inactivation of the ribonucleic acid-processing enzyme ribonuclease E blocks cell division

The Escherichia coli endoribonuclease ribonuclease E, the enzyme responsible for the processing of precursor 5S ribosomal ribonucleic acid (RNA) from the nascent ribosomal RNA transcript, is thermolabile in rne-3071 mutants. When cells of such a strain were shifted to a nonpermissive temperature, the function of ribonuclease E was almost instantaneously inactivated. However, a threefold linear increase in absorbance took place over a 3-h period, and similar linear increases occurred in all the macromolecules we measured, including deoxyribonucleic acid, RNA, protein, and lipopolysaccharides. Interestingly, during this period, the cells elongated but failed to divide. Thus, these experiments suggest that an early effect of inactivation of the RNA processing enzyme ribonuclease E is a block in cell division.