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Molecular characterization of the D surface protein gene subfamily in Paramecium primaurelia
F M Bourgain-Guglielmetti1, F M Caron
1Laboratoire de Génétique Moléculaire, Ecole Normale Supérieure, Paris, France.
Abstract:
When paramecium primaurelia expresses the D serotype, a major high molecular weight mRNA species is detected in the cytoplasm. Using the cDNA derived from this mRNA as a probe, three very similar genes, D alpha, D beta and D gamma, were cloned. Of these three genes, we show that only the D alpha mRNA is present in the cytoplasm of cells expressing the D serotype and corresponds to the major mRNA species. The nucleotide sequence of the entire coding region of the D alpha gene, as well as the upstream and downstream sequences, has been determined. The 7632-nucleotide open reading frame encodes a putative protein that displays the characteristic cysteine residue periodicity of Paramecium surface antigens but does not contain central tandemly repeated sequences. Partial sequences of the two nonexpressed genes D beta and D gamma indicate a high percentage of identity (90%-95%) with the D alpha gene, suggesting that D beta and D gamma genes are either very similar surface protein genes whose transcription is repressed trough mutual exclusion, or perhaps are pseudogenes. A region of variable DNA rearrangement was identified 1 kb upstream of the D gamma gene. This macronuclear region arises from the same micronuclear locus by alternative excision of internal eliminated sequences during macronuclear development.
Insights
Researchers identified the D alpha gene in Paramecium primaurelia, responsible for producing a major mRNA species in D serotype cells. This gene encodes a surface antigen protein, with related D beta and D gamma genes possibly being pseudogenes or repressed genes.
Area of Science:
- Molecular Biology
- Genetics
- Cell Biology
Background:
- Paramecium primaurelia exhibits different surface antigens, known as serotypes.
- The D serotype is associated with a specific high molecular weight mRNA in the cytoplasm.
Purpose of the Study:
- To clone and characterize the genes responsible for the D serotype in Paramecium primaurelia.
- To determine the nucleotide sequence and protein-encoding potential of the D alpha gene.
Main Methods:
- cDNA hybridization to identify and clone homologous genes.
- Nucleotide sequencing of the D alpha gene and flanking regions.
- Comparative sequence analysis of D alpha, D beta, and D gamma genes.
Main Results:
- Three highly similar genes (D alpha, D beta, D gamma) were cloned.
- Only D alpha mRNA was detected in D serotype cells, corresponding to the major cytoplasmic mRNA.
- The D alpha gene encodes a putative surface antigen protein with characteristic cysteine periodicity.
- D beta and D gamma genes show high sequence identity to D alpha but are not expressed, suggesting they may be pseudogenes or subject to repression.
- Variable DNA rearrangement upstream of the D gamma gene was identified.
Conclusions:
- The D alpha gene is the expressed gene encoding the major mRNA species for the D serotype in Paramecium primaurelia.
- The D beta and D gamma genes are closely related to D alpha, potentially representing pseudogenes or genes regulated by mechanisms like mutual exclusion.
- Macronuclear development involves DNA rearrangement, as evidenced by the region upstream of the D gamma gene.