Two two-gene macronuclear chromosomes of the hypotrichous ciliatesOxytricha fallax andO. trifallax generated by alternative processing of the 81 locus

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Abstract
We describe the first known macronuclear chromosomes that carry more than one gene in hypotrichous ciliated protozoa. These 4.9‐ and 2.8‐kbp chromosomes each consist almost exclusively of two protein‐coding genes, which are conserved and transcribed. The two chromosomes share a common region that consists of a gene that is a member of the family of mitochondrial solute carrier genes (CR‐MSC; [Williams and Herrick (1991): Nucleic Acids Res 19:4717–4724]. Each chromosome also carries another gene appended to its common region: The 4.9‐kbp chromosome also carries a gene that encodes a protein that is rich in glutamine and charged amino acids and bears regions of heptad repeats characteristic of coiled‐coils. Its function is unknown. The second gene of the 2.8 kbp chromosome is a mitochondrial solute carrier gene (LA‐MSC); thus, the 2.8‐kbp chromosome consists of two mitochondrial solute carrier paralogs. Phylogenetic analysis indicates that the two genes were duplicated before ciliates diverged from the main eukaryotic lineage and were subsequently juxtaposed. The CR‐ and LA‐MSC genes are each interrupted by three introns. The introns are not in homologous positions, suggesting that they may have originated from multiple group II intron transpositions. These chromosomes and their genes are encoded in the Oxytricha germline by the 81 locus. This locus is alternatively processed to generate a nested set of three macronuclear chromosomes, the 4.9‐ and 2.8‐kbp chromosomes and a third (1.6 kbp) which consists almost exclusively of the shared common gene, CR‐MSC. Such alternative processing is common in macronuclear development of O. fallax [Cartinhour and Herrick (1984): Mol Cell Biol 4:931–938]. Possible functions for alternative processing are considered; e.g., it may serve to physically link genes to allow co‐regulation or co‐replication by a common cis‐acting sequence. Dev. Genet. 20:348–357, 1997.