Transcript profiling was used to look for genes that differ in expression between the SAH hydrolase deficient and hypomethylated hog1-1 mutant and the parental (HOG1) line. This analysis identified a subset of gene transcripts that were up-regulated in hog1-1 plants. The majority of these transcripts were from genes located in the pericentromeric heterochromatin. About a third of the genes are annotated as transposons or having transposon homology. Subsequent experiments using Northern blots, RT-PCR and real-time RT-PCR confirmed the up-regulation of 19 of the genes and identified a set of molecular probes for genes that are up-regulated in the hog1-1 background. Six (of six genes tested) of the hog1-1 up-regulated genes are also up-regulated in the hypomethylated ddm1 mutant, three in the hypomethylated met1 mutant and three in the dcl3 mutant. The results suggest that the hypomethylation in the mutant lines may have a causal role in the up-regulation of these transcripts.

Three Agrobacterium tumefaciens mutants with chromosomal mutations that affect bacterial virulence were isolated by transposon mutagenesis. Two of the mutants were avirulent on all hosts tested. The third mutant, Ivr-211, was a host range mutant which was avirulent on BryophyUum diagremontiana, Nicotiana tabacum, N. Infection of susceptible dicotyledonous plants by Agrobacterium tumefaciens results in crown gall tumor formation. The mechanism by which the bacterium infects the plant is not fully understood. An early step in tumor formation involves the attachment of the bacterium to host plant cells (12). Once attached to the plant, the bacterium begins a process that results in the movement of tumor-inducing (Ti) plasmid DNA sequences from the bacterium into the plant host genome (2, 28). Depicker et al. have identified the DNA sequences transferred and termed them T-DNA (5). Without successful transfer of the T-DNA from the bacterium to the plant, tumor formation cannot occur. This paper examines the properties of three avirulent TnS insertion mutants of A. tumefaciens,. These TnS insertions are contained within the chromosome of the bacterium; thus, the insertions have no direct affect on the vir region of the Ti plasmid which is responsible for T-DNA transfer. The only previously identified chromosomal genes required for virulence are chvA and chvB (6), att (16), pscA (27) (also called exoC [1]), chvD (32), and chvE (9). Mutations in each of the first four genes listed result in the inability of the bacteria to attach to host cells and the consequent loss of bacterial virulence. Mutations in chvD result in reduced virulence on Bryophyllum diagremontiana (visible tumors were still formed) and reduced induction of virG by acetosyringone (32). Transposon insertions in chvE result in a * Corresponding author.