Structure-function analysis of interallelic complementation in ROOTY transheterozygotes

Auxin is a crucial plant growth regulator. Forward genetic screens for auxin-related mutants have led to the identification of key genes involved in auxin biosynthesis, transport, and signaling. Loss-of-function mutations in the genes involved in indole glucosinolate biosynthesis, a metabolically-related route that produces defense compounds from indolic precursors shared with the auxin pathway, result in auxin overproduction. We identified an allelic series of fertile, hypomorphic mutants for an essential indole glucosinolate route gene ROOTY (RTY) that show a range of high-auxin defects. Genetic characterization of these lines uncovered phenotypic suppression by cyp79b2 b3, wei2, and wei7 mutants and revealed the phenomenon of interallelic complementation in several RTY transheterozygotes. Structural modeling of RTY shed light on the structure-to-function relations in the RTY homo- and heterodimers and unveiled the likely structural basis of interallelic complementation. This work underscores the importance of employing true null mutants in genetic complementation studies.

Interallelic complementation provides functional evidence for cohesin–cohesin interactions on DNA

The cohesin complex (Mcd1p, Smc1p, Smc3p, and Scc3p) has multiple roles in chromosome architecture, such as promoting sister chromatid cohesion, chromosome condensation, DNA repair, and transcriptional regulation. The prevailing embrace model for sister chromatid cohesion posits that a single cohesin complex entraps both sister chromatids. We report interallelic complementation between pairs of nonfunctional mcd1 alleles (mcd1-1 and mcd1-Q266) or smc3 alleles (smc3-42 and smc3-K113R). Cells bearing individual mcd1 or smc3 mutant alleles are inviable and defective for both sister chromatid cohesion and condensation. However, cells coexpressing two defective mcd1 or two defective smc3 alleles are viable and have cohesion and condensation. Because cohesin contains only a single copy of Smc3p or Mcd1p, these examples of interallelic complementation must result from interplay or communication between the two defective cohesin complexes, each harboring one of the mutant allele products. Neither mcd1-1p nor smc3-42p is bound to chromosomes when expressed individually at its restrictive temperature. However, their chromosome binding is restored when they are coexpressed with their chromosome-bound interallelic complementing partner. Our results support a mechanism by which multiple cohesin complexes interact on DNA to mediate cohesion and condensation.