GENOMIC AND TRANSCRIPTIONAL LINKAGE OF THE GENES FOR CALMODULIN, EF-HAND 5-PROTEIN, AND UBIQUITIN EXTENSION PROTEIN-52 IN TRYPANOSOMA-BRUCEI

We report genomic linkage of a pair of tandem, identical ubiquitin-extension protein 52 (EP52) genes, a novel EF-hand superfamily member gene (EFH5), and the calmodulin gene cluster in Trypanosoma brucei. The intergenic regions of these four genes are short: about 108 bp between the calmodulin gene C and the EFH5 gene, about 111 bp between the EFH5 gene and the ubiquitin-EP52/1 gene, and about 116 bp between the ubiquitin-EP52/1 and -EP52/2 genes. RNA molecules that span these three intergenic regions have been detected by polymerase chain reaction, which suggests that the genes are transcribed in a polycistronic manner. Transcription of the calmodulin, EFH5, and ubiquitin-EP52 genes in isolated nuclei is rapidly inactivated by UV irradiation, which further strengthens the hypothesis that this cluster of three different genes is transcribed in a polycistronic manner and suggests that they are under the control of a single distant upstream promoter. These results suggest that polycistronic transcription is common in trypanosomes and will probably be found for most, if not all, protein-encoding genes. The presence of at least three housekeeping genes with different known or potential regulatory functions within a polycistronic unit suggests that regulation of transcription initiation plays an important role in the coordinated expression of housekeeping genes in trypanosomes.