Different genomic regions replicate at a distinct time during S-phase. The SuUR mutation alters replication timing and the polytenization level of intercalary and pericentric heterochromatin in Drosophila melanogaster salivary gland polytene chromosomes. We analyzed SuUR in different insects, identified conserved regions in the protein, substituted conserved amino acid residues, and studied effects of the mutations on SUUR function. SuUR orthologs were identified in all sequenced drosophilids, and a highly divergent ortholog was found in the mosquito genome. We demonstrated that SUUR evolves at very high rate comparable with that of Transformer. Remarkably, upstream ORF within 5' UTR of the gene is more conserved than SUUR in drosophilids, but it is absent in the mosquito. The domain structure and charge of SUUR are maintained in drosophilids despite the high divergence of the proteins. The N-terminal part of SUUR with similarity to the SNF2/SWI2 proteins displays the highest level of conservation. Mutation of two conserved amino acid residues in this region impairs binding of SUUR to polytene chromosomes and reduces the ability of the protein to cause DNA underreplication. The least conserved middle part of SUUR interacting with HP1 retains positively and negatively charged clusters and nuclear localization signals. The C terminus contains interlacing conserved and variable motifs. Our results suggest that SUUR domains evolve with different rates and patterns but maintain their features.