Compensatory Regulation of Two Closely Related Hemoglobin Loci in CHIRONOMUS TENTANS.

Regulatory mutants in the hemoglobin system of Chironomus larvae can be detected by shifts in the relative quantity of specific components among the 10-12 mixed monomers. One such mutant, at or near a hemoglobin structural locus in the right tip of chromosome 3, greatly increases the quantity of that minor hemoglobin and greatly diminishes the quantity of the hemoglobin normally most abundant. Heterozygotes for the mutant are quantitatively intermediate, suggesting a transcriptional basis.-Structural similarities of the two hemoglobins indicate a close evolutionary relationship, and their interdependent but non-coordinate regulation is interpreted as competition for a common factor which functions in transcription. If evolutionary duplication of both structural genes and promotors is assumed, this factor may be a sigma subunit of RNA polymerase which recognizes similar but non-identical promotor regions. Parallels in the compensatory control of human hemoglobins are described.