The lethal phenotype caused by null mutations in the Escherichia coli htrB gene is suppressed by mutations in the accBC operon, encoding two subunits of acetyl coenzyme A carboxylase.

Insertion mutations in the Escherichia coli htrB gene result in the unique phenotype of not affecting growth at temperatures below 32.5 degrees C but leading to a loss of viability at temperatures above this in rich media. When htrB bacteria growing in rich media were shifted to the nonpermissive temperature of 42 degrees C, they continued to grow at a rate similar to that at 30 degrees C but they produced phospholipids at the rate required for growth at 42 degrees C. This led to the accumulation of more than twice as much phospholipid per milligram of protein compared with that in wild-type bacteria. Consistent with HtrB playing a role in phospholipid biosynthesis, one complementation group of spontaneously arising mutations that suppressed htrB-induced lethality were mapped to the accBC operon. This operon codes for the biotin carboxyl carrier protein and biotin carboxylase subunits of the acetyl coenzyme A carboxylase enzyme complex, which catalyzes the first step in fatty acid biosynthesis. Four suppressor mutations mapped to this operon. Two alleles were identified as mutations in the accC gene, the third allele was identified as a mutation in the accB gene, and the fourth allele was shown to be an insertion of an IS1 transposable element in the promoter region of the operon, resulting in reduced transcription. The suppressor mutations caused a decrease in the rate of phospholipid biosynthesis, restoring the balance between the biosynthesis of phospholipids and growth rate, thus enabling htrB bacteria to grow at high temperatures.