Isolation and characterization of mRNA from Paramecium aurelia

Deviation from the universal code shown by the gene for surface protein 51A in Paramecium

The immobilization antigens (i-antigens) of Paramecium, large polypeptides of relative molecular mass approximately 300,000, are located on the cell surface. Each i-antigen is encoded by a different unlinked gene, and no more than one gene is expressed at a time. The proteins and the mRNAs and genes encoding them are readily isolated. Here we report the nucleotide sequence of three regions of the A i-antigen gene from stock 51 of Paramecium tetraurelia. Surprisingly, all reading frames contain TAA and TAG stop codons, even though there is evidence that one reading frame of these sequences codes for the i-antigen. We suggest that in Paramecium UAA and UAG code for amino acids, instead of serving as translational stops as they do in all other organisms.

Does Paramecium primaurelia use a different genetic code in its macronucleus?

It has long been known that messenger RNAs (mRNAs) of ciliates and in particular of Paramecium are not translated well in heterologous in vitro translation systems. Recently, we have demonstrated for Paramecium primaurelia that this phenomenon results from the presence of well-defined blocking sites in the coding sequences of almost all mRNAs, and that these sites are an intrinsic feature of the primary as opposed to the secondary structure of the mRNAs. Here we show that both the gene and the mRNA for the G surface antigen of P. primaurelia contain numerous TAA and TAG codons scattered throughout their coding sequences. We propose that these codons do not represent termination codons in P. primaurelia but instead code for glutamic acid or glutamine and that the in vitro translation of Paramecium mRNAs is blocked by their presence.

The effect of temperature change on gene expression in Paramecium primaurelia

When paramecia grown at 24 degrees C are transferred rapidly to 32 degrees C, DNA and protein synthesis continue uninterrupted but at higher rates. Electron microscopic observations indicate that more of the macronuclear chromatin is transcribed at the elevated temperature. This interpretation is supported by hybridization experiments which show that the percentage of the macronuclear genome transcribed into poly(A)+RNA is 24 degrees C and 35% at 32 degrees C. Kinetic analysis of cDNA-poly(A)+RNA hybridizations reveals three abundance classes of poly(A)+RNA and indicates that the number of genes expressing low abundance sequences is about 9000 at 24 degrees C and 13000 at 32 degrees C. The intermediately abundant and highly abundant classes are represented by 100-200 and 1-3 different kinds of RNA sequence, respectively. Cross hybridization shows that changes occur throughout the distribution of abundance classes of poly(A)+RNA with increase in temperature.

Macronuclear DNA organization and transcription in Paramecium primaurelia

Macronuclear DNA was isolated from Paramecium primaurelia, stock 168. Although the macronucleus is polyploid to the extent of 840C, in other respect the DNA appears to be simply organized, having neither satellite sequences nor substantial amounts of intermediately repetitive sequence. The sequence complexity of macronuclear DNA is quite low for a eukaryote cell, being approximately 19 times more complex than the genome of Escherichia coli. In addition, the GC content is low (25%) and the isolated DNA molecules have lengths mostly in the range 0.2-5 micrometer. In these various respects, the macronuclear DNA of Paramecium is similar to that of other ciliates. A clone of Paramecium cultured under controlled conditions contains polyadenylated RNA sequences which are homologous to 5-8% of the macronuclear DNA. Sequence complexity analysis indicates that the polyadenylated RNA contains two abundance classes of molecules, one present at low frequency and transcribed from approximately 10(4) genes, the other at 100 times greater concentration and transcribed from about 100 genes. The relevance of these results to the control of gene expression in Paramecium is discussed.