The utilization of exogenous fatty acids by wild-type Escherichia coli K-12 has been studied using [14C]acetate incorporation to monitor fatty acid synthesis and unusual fatty acid analogs as a nonradioactive supplement to distinguish acyl groups in the phospholipid derived from the exogenous source. Certain strains are found to regulate synthesis of saturated and unsaturated fatty acids in response to exogenous fatty acid supplements. This effect is associated with incorporation of the exogenous supplement into phospholipid. Even and odd chain-length saturated, trans and cis unsaturated fatty acids were included among the fatty acid supplements examined. Although a principal result of the incorporation of the exogenous fatty acid is a replacement of structurally related acyl groups, the effect is pleotropic in many instances: for example, 16:0 replaces 18:1 and vise versa. These observations can be rationalized in terms of the positional distribution in the phospholipid of the acyl residues derived from synthesis and the pattern of incorporation of the exogenous fatty acids. β-OH-14:0 and-12:0, added to cultures as supplements, are not detectably incorporated into complex lipid (would be predominantly lipid A) and show little or no influence on fatty acid synthesis.
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