Occurrence of closely spaced genes in the nuclear genome of the agarophyteGracilaria gracilis

Little is known about the structure and organisation of nuclear genomes in red algae. In particular, it is not known whether genes are densely or loos ely packed, whether gene order is conserved, whether their genes tend to oc cur in one or multiple copies and whether their nuclear genes tend to be co mpact or interrupted by numerous introns. Sequencing of cloned genomic DNA from Gracilaria gracilis has begun to provide provisional answers to some o f these questions. Four pairs of closely spaced genes have been found in G. gracilis upon sequencing genomic clones that contain genes for UDPglucose pyrophosphorylase, galactose-1-phosphate uridylyltransferase, the beta subu nit of tryptophan synthetase, and methionine sulphoxide reductase (a fifth pair of closely spaced genes, encoding polyubiquitin and aconitase, was rep orted earlier). An open reading frame with significant similarity to anothe r known gene occurs close (< 1.7 kbp) to each of these genes. In two pairs the intergenic region is less than 400 bp in length, and for these the loca tion of the putative polyadenylation signals indicates that the gene transc ripts, encoded on opposite strands, have overlapping (hence complementary) 3' regions. These somewhat unexpected findings begin to establish a basis f or genome-level characterisation of red algae.