Cutting, splicing, reordering, and elimination of DNA sequences in hypotrichous ciliates

Hypotrichous ciliates extensively process genomic DNA during their life cycle. Processing occurs after cell mating, beginning with multiple rounds of DNA replication to form polytene chromosomes. Thousands of transposonlike elements are then excised from the chromosomes and destroyed, and thousands of short, internal eliminated sequences (IESs) are excised from coding and noncoding parts of genes and destroyed. IES removal from a gene is accompanied by splicing of the remaining chromosomal DNA segments to form a transcriptionally competent gene. For some genes these DNA segments are in a scrambled order and are ligated into a genetically correct order at the time of IES removal. Next the polytene chromosomes are cut up band‐by‐band and all genes are excised from the chromosomes as short, linear molecules averaging 2.2 kbp (in Oxytricha nova). Gene excision is accompanied by destruction of all nongenic DNA, which, together with the transposonlike elements and IESs, accounts for ∼95% of the total sequence complexity of the genome in O. nova. Telomeric sequences are added to the excised gene‐sized DNA molecules. Finally, the gene‐sized molecules are replicated several times to form the macronucleus of the organism.