Factors affecting relative proportions of long-chain fatty acids produced by the growing cells ofBacillus cereushave been studied. The organism produces nine branched (i-C12,i-C13,i-C14,i-C15,i-C16,i-C17,a-C13,a-C15, anda-C17), two normal (n-C14andn-C16), and four monounsaturated (i-C161−,i-C17−1,a-C171−, andn-C161−), in addition to some minor fatty acids. On the basis of biosynthetic relationships these are grouped into four series: anteiso odd-numbered, iso odd-numbered, iso even-numbered, and normal even-numbered. The proportion of the normal series of fatty acids is generally not changed by varying growth conditions. The proportions of the three series of branched-chain fatty acids, however, are greatly affected depending upon growth phase and culture medium used. Younger cells tend to produce the four series in a closer ratio (1.5/1.8/0.8/1.0), whereas older cells produce four series in a wider ratio (1/2.2/0.8/1.0).The precursors of the terminal portions of branched-chain fatty acids,L-leucine,L-isoleucine,L-valine, and the related substrates, affected greatly the ratio of the three series of branched-chain fatty acids. Apparently the synthesis of iso odd-numbered acids is more tightly controlled than the synthesis of the two other series of fatty acids. Data suggest that this regulation is very likely to be due to the substrate specificity of the enzyme(s) involved in the fatty acid synthesis, rather than the relative availability of the precursors.
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