Structure and sequence divergence of two archaebacterial genes.

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Abstract
The DNA sequences of a region that includes the hisA gene of 2 related methanogenic arachaebacteria, Methanococcus voltae and Methanococcus vannielii, were compared. Both organisms show a similar genome organization in this region, displaying 3 open reading frames (ORF) separated by regions of very high A + T content. Two of the ORF, including ORFHisA, show significant DNA sequence homology. As might be expected for organisms having a genome that is A + T-rich, there is a high preference for A and U as the 3rd base in codons. Although the regions upstream of the structural genes contain prokaryotic-like promoter sequences, it is not unknown whether they are recognized as promoters in these archaebacterial cells. A ribosome binding site, G-G-T-G, is located 6 base pairs preceding the ATG translation initiation sequence of both hisA genes. The sequences upstream of the 2 hisA genes show only limited sequence homology. The M. voltae intergenic contains 4 tandemly arranged repetitions of an 11-base-pair sequence, whereas the M. vannielii sequence contains both direct and inverted repetitive sequences. Based on the degree of hisA sequence homology, it is concluded that M. voltae and M. vannielii are closely related taxonomically than are members of the enteric group of eubacteria.

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