|
The nucleotide sequence of the sucB gene, which encodes the dihydrolipoamide
succinyltransferase component (E2o) of the 2-oxoglutarate dehydrogenase complex
of Escherichia coli K12, has been determined by the dideoxy chain-termination
method. The results extend by 1440 base pairs the previously reported sequence
of 3180 base pairs, containing the sucA gene. The sucB structural gene comprises
1209 base pairs (403 codons excluding the initiating AUG), and it is preceded by
a 14-base-pair intercistronic region containing a good ribosomal binding site.
The absence of a typical terminator sequence and the presence of an IS-like
sequence downstream of sucB suggest that there may be further gene(s) in the suc
operon. The IS-like sequence is homologous with other intercistronic sequences
including that between the sdhB and sucA genes, the overall gene organisation
being: sdhB-IS-sucAsucB-IS-. The patterns of codon usage indicate that sucB may
be more strongly expressed than sucA, consistent with the disproportionate
contents of their products in the oxoglutarate dehydrogenase complex. The
predicted amino acid composition and Mr (43 607) of the succinyltransferase
component agree with previous studies on the purified protein. Comparison with
the corresponding acetyltransferase component of the pyruvate dehydrogenase
complex (E2p, aceF gene product) indicates that each contains two analogous
domains, an amino-terminal lipoyl domain linked to a carboxy-terminal catalytic
and subunit binding domain. The lipoyl domain of the acetyltransferase (E2p)
comprises three tandemly repeated approximately 100-residue lipoyl binding
regions containing two short (approximately 19 residues) internal repeats,
whereas the lipoyl domain of the succinyltransferase (E2o) contains just one
approximately 100-residue lipoyl binding region, with approximately 27% homology
to each of the three comparable regions in E2p, and no detectable internal
repeats. The catalytic and subunit binding domains, each approximately 300
residues, have an overall homology of 34% and, consistent with their combination
of analogous and specific functions, some regions are more homologous than
others. Both sequences feature segments rich in proline and alanine. In E2p
these occur at the carboxy-terminal ends of each of the three lipoyl binding
regions, there being a particularly extended sequence at the end of the third
repeat, whereas in E2o the main proline-alanine segment is found approximately
50 residues into the subunit binding domain.(ABSTRACT TRUNCATED AT 400 WORDS)
|
