Transcription of the Archaeal trkA Homolog in

Transcription of the archaeal trkA gene homolog in Methanosarcina mazei S-6 was studied at the optimal growth temperature of 37&C and after heat shock at 45&C. Northern (RNA) blotting results (transcript size) and data from primer extension experiments to map the transcription initiation site indicate that trkA is cotrans-cribed with another gene. The latter, orf11, encodes a protein of 94 amino acids (10,611 Da) and is located upstream of trkA, with which it overlaps: the translation stop codon of orf11, TGA, shares the bases T and G with the translation start codon of trkA, ATG. These genes ’ transcription was decreased by heat shock to the point of making the transcript undetectable by Northern or dot blotting procedures. orf11 and trkA differ in codon usage patterns, and the proteins coded by them, i.e., Orf11 and TrkA, are dissimilar in amino acid sequence and composition. The only archaeal trkA gene homolog known so far is from the anaerobic methanogen Methanosarcina mazei S-6 and was described in 1993 (17). At that time, there was only one trkA gene sequence in the databases, and it was from the eubacte-rium Escherichia coli (36). More recently, two additional ho-mologs have been found also in eubacteria: one in Vibrio algi-nolyticus (24), and the other, named sapG, in Salmonella typhimurium (30). The physiological role of this gene’s product, TrkA in E. coli and V. alginolyticus and SapG in S. typhi-murium, has been studied, and it is understood to a consider