Transfer-RNA genes in archaea often have introns intervening between exon sequences. The structural motif at the boundary between exon and intron is the bulge-helix-bulge. Computational investigations of these boundary Structures in Haloarcula marismortui lead us to propose that tRNA-isoleucine and tRNA-tryptophan genes are co-located. Precise in silico identification of the splice-sites on the bulges at the exon-intron boundaries lead us to infer that a single intron-containing composite tRNA-gene can give rise to more than one gene product. (c) 2005 Elsevier Inc. All rights reserved.