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Good things in small packages: The tiny genomes of chlorarachniophyte endosymbionts
Author(s) -
Gilson Paul R.,
McFadden Geoffrey I.
Publication year - 1997
Publication title -
bioessays
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.175
H-Index - 184
eISSN - 1521-1878
pISSN - 0265-9247
DOI - 10.1002/bies.950190212
Subject(s) - biology , genome , genetics , genome size , gene , ribosomal rna
Chlorarachniophytes are amoeboflagellate, marine protists that have acquired photosynthetic capacity by engulfing and retaining a green alga. These green algal endosymbionts are severely reduced, retaining only the chloroplast, nucleus, cytoplasm and plasma membrane. The vestigial nucleus of the endosymbiont, called the nucleomorph, contains only three small linear chromosomes and has a haploid genome size of just 380 kb ‐ the smallest eukaryotic genome known. Initial characterisation of nucleomorph DNA has revealed that all chromosomes are capped with inverted repeats comprising a telomere and a single ribosomal RNA operon. The nucleomorph genome is the quintessence of compactness; average space between genes is a mere 65 bp, some genes overlap, others are cotranscribed. Intense reductive pressures upon nucleomorph genes have apparently squeezed their spliceosomal‐type introns down to only 18, 19 or 20 bases in length. Studies to date indicate the nucleomorph ‐ essentially a stripped‐down eukaryotic genome ‐ encodes principally genetic housekeeping functions such as translation, transcription and splicing.