Expression of the gene encoded by a family of macronuclear chromosomes generated by alternative DNA processing in...

Hypotrichous ciliated protozoa, such as Oxytricha fallax, produce tiny chromosomes during generation of the transcriptionally active macronucleus. The 81-MAC family of macronuclear chromosomes is produced by alternative DNA processing, such that the chromosomes share a common region of 1.6 kbp. Transcription of a 1.3 kb mRNA from the common region has been analyzed. Transcription starts very near the telomere (34 bp), in a 23 bp region of pure A + T DNA. Polyadenylation sites are very near the other telomere (26 bp), also in a region of nearly pure A + T DNA. Three introns are clustered in the first third of the gene. Intron removal can follow polyadenylation, and the order of removal is not fixed. All three known sequence versions of the 81-MAC chromosomes are represented in the mRNA pool, with no evidence of any further versions. The A + T sequences surrounding the transcription starts and polyadenylation sites are conserved among versions. Introns have conserved 5' and 3' ends and a putative branch-point sequence (YYRAT), but otherwise are highly diverged and are AT-rich. A single long open reading frame, interrupted by the three introns, encodes a homolog of known mitochondrial solute carriers, and contains the codon TAA, which does not encode 'stop,' but a conserved glutamine; TAG appears also to encode glutamine. The results significantly enlarge the small data set of transcription start and polyadenylation sites, of intron features, and of translation signals for hypotrichs.