EXPRESSION OF THE RHODOBACTER-SPHAEROIDES HEMA AND HEMT GENES, ENCODING 2 5-AMINOLEVULINIC ACID SYNTHASE ISOZYMES

被引:98
作者
NEIDLE, EL [1 ]
KAPLAN, S [1 ]
机构
[1] UNIV TEXAS,HLTH SCI CTR,DEPT MICROBIOL & MOLEC GENET,POB 20708,HOUSTON,TX 77225
关键词
D O I
10.1128/JB.175.8.2292-2303.1993
中图分类号
Q93 [微生物学];
学科分类号
071005 ; 100705 ;
摘要
The nucleotide sequences of the Rhodobacter sphaeroides hemA and hemT genes, encoding 5-aminolevulinic acid (ALA) synthase isozymes, were determined. ALA synthase catalyzes the condensation of glycine and succinyl coenzyme A, the first and rate-limiting step in tetrapyrrole biosynthesis. The hemA and hemT structural gene sequences were 65% identical to each other, and the deduced HemA and HemT polypeptide sequences were 53% identical, with an additional 16% of aligned amino acids being similar. HemA and HemT were homologous to all characterized ALA synthases, including two human ALA synthase isozymes. In addition, they were evolutionarily related to 7-keto-8-aminopelargonic acid synthetase (BioF) and 2-amino-3-ketobutyrate coenzyme A ligase (Kbl), enzymes which catalyze similar reactions. Two hemA transcripts were identified, both expressed under photosynthetic conditions at levels approximately three times higher than those found under aerobic conditions. A single transcriptional start point was identified for both transcripts, and a consensus sequence at this location indicated that an Fnr-like protein may be involved in the transcriptional regulation of hemA. Transcription of hemT was not detected in wild-type cells under the physiological growth conditions tested. In a mutant strain in which the hemA gene had been inactivated, however, hemT was expressed. In this mutant, hemT transcripts were characterized by Northern (RNA) hybridization, primer extension, and ribonuclease protection techniques. A small open reading frame of unknown function was identified upstream of, and transcribed in the same direction as, hemA.
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页码:2292 / 2303
页数:12
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