Three group I introns of bacteriophage T4 have been compared with respect to their sequence and structural properties. The introns include the td intervening sequence, as well as the two newly described introns in the nrdB and sunY genes of T4. The T4 introns are very closely related, containing phylogenetically conserved sequence elements that allow them to be folded into a core structure that is characteristic of eukaryotic group IA introns. Similarities extend outward to the exon sequences surrounding the three introns. All three introns contain open reading frames (ORFs). Although the intron ORFs are not homologous and occur at different positions, all three ORFs are looped-out of the structure models, with only the 3' ends of each of the ORFs extending into the secondary structure. This arrangement invites interesting speculations on the regulation of splicing by translation. The high degree of similarity between the T4 introns and the eukaryotic group I introns must reflect a common ancestry, resulting either from vertical acquisition of a primordial RNA element or from horizontal transfer.The discovery of catalytic properties of RNA molecules has strengthened the notion that RNA was the primordial nucleic acid (1). These RNAs are exemplified by introns in fungal mitochondria and the large rRNA of Tetrahymena (reviewed in ref. 2). Placed into two groups based on conserved features of sequence and secondary structure (3), the group I and group II introns are excised by related yet distinct mechanisms, which can sometimes proceed autocatalytically (2). Self-splicing introns also exist in at least three genes of bacteriophage T4 (4-6). These T4 mRNA precursors are spliced by a mechanism resembling that used by group I introns of eukaryotes, involving 5' guanosine addition and circularization of the intron (7-9). Since all of the information for both catalysis and specificity of splicing is contained in the precursor RNA, it is of interest to compare the structural properties of these functionally similar RNAs from such phylogenetically distant organisms. Examination of these structures may also give clues to possible regulatory roles played by these introns in gene expression.RESULTS AND DISCUSSION Splice Junction of the nrdB Gene. Gott et al. (5) described the existence of two T4 introns, one of which mapped to nrdB, the gene for the small subunit of ribonucleoside diphosphate reductase. Sjoberg et al. (6) have also inferred an intron in nrdB by nucleotide sequence analysis. By comparing homologies with the small subunit of ribonucleoside diphosphate reductase from Escherichia coli, they concluded that two coding sequences were separated by about 600 base pairs (bp) of nonhomologous DNA. This corresponded closely to the size determined for the intron that could be autocatalytically excised from RNA from the nrdB gene (5).We have determined the precise boundaries of the RNAprocessing event by primer-extension sequencing of RNA extracted from cells infected by phage T4 (Fig. 1A). This RNA contains bot...

Bacteriophage T4 alpha- and beta-glucosyltransferases link glucosyl units to the 5-HMdC residues of its DNA. The monoglucosyl group in alpha-linkage predominates over the one in beta linkage. Having recently reported on the nucleotide sequence of gene alpha gt (1) we now determined the nucleotide sequence of gene beta gt. The genes were each cloned on a high expression vector under the control of the lambda pL promoter. After thermo-induction the proteins were isolated and purified to homogeneity. To verify that the translational starting sites and the proposed reading frames are effective in vivo the sequence of the first 31 amino acid residues from gp alpha gt and the first 30 amino acid residues from gp beta gt were determined by Edman degradation. The primary structures of the two proteins seem to have only limited structural similarities. The results are discussed comparing secondary structure predictions and homologies with other proteins from the protein sequence database of the Protein Identification Resource.

This sequence comprises 19 open reading frames (orfs) including T4 genes 49 and nrdC which were identified (1). The nomenclature of the orfs within this sequence is in agreement with the T4 genomic map (2).

Group I self-splicing introns are present in the td, nrdB and sunY genes of bacteriophage T4. We previously reported that whereas the td intron is present in T2, T4 and T6, the nrdB intron is present in T4 only. These studies, which argue in favor of introns as mobile genetic elements, have been extended by defining the distribution of all three T4 introns in a more comprehensive collection of T2, T4 and T6 isolates. The three major findings are as follows: First, all three introns are inconsistently distributed throughout the T-even phage family. Second, different T2 isolates have different intron complements, with T2H and T2L having no detectable introns. Third, the intron open reading frames are inherited or lost as a unit with their respective flanking intron core elements. Furthermore, exon sequences flanking sites where introns are inserted in the T4 td, sunY and nrdB genes were determined for all the different T-even isolates studied. Six of eighteen residues surrounding the junction sequences are identical. In contrast, a comprehensive comparison of exon sequences in intron plus and intron minus variants of the sunY gene indicate that sequence changes are concentrated around the site of intron occurrence. This apparent paradox may be resolved by hypothesizing that the recombination events responsible for intron acquisition or loss require a consensus sequence, while these same events result in sequence heterogeneity around the site.

In Escherichia coli, proteins with abnormal conformations, protein fragments, and some foreign proteins encoded by cloned genes are turned over rapidly as compared with normal cellular proteins (10,12,17,20,40). The degradation of such abnormal proteins is ATP dependent (12), and the Lon protease, a tetramer of 94-kilodalton subunits with ATP-dependent proteolytic activity, has a major role in this process (5,6,11,14,34