The vertebrate high-mobility-group (HMG) protein HMG1 is an abundant non-histone protein which is considered as an architectural element in chromatin. In the monocotyledonous plant maize, four different HMG1-like proteins (HMGa, HMGc1/2, HMGd) have been identified, whereas other eukaryotes usually express only two different proteins of this type. We have examined here the HMG1-like proteins of the dicotyledonous plant Arabidopsis thaliana. The isolation and analysis of cDNAs encoding five different so far uncharacterised HMG1-like proteins (now termed HMG alpha, HMG beta1/2, HMG gamma, HMG delta) from Arabidopsis indicates that the expression of multiple HMG1-like proteins is a general feature of (higher) plants. The Arabidopsis HMG1-like proteins contain an HMG domain as a common feature, but outside this conserved DNA-binding motif the amino acid sequences are significantly different indicating that this protein family displays a greater structural variability in plants than in other eukaryotes. The five HMG1-like proteins were expressed in Escherichia coli and purified. They bind with somewhat different affinity to linear double-stranded DNA. The recognition of DNA structure is evident from their preferential interaction with DNA minicircles relative to linear DNA. Reverse-transcribed PCR suggested that the five HMG1-like genes are simultaneously expressed in Arabidopsis leaves and suspension culture cells.