SummaryThe integrase (Int) proteins encoded by bacteriophages HK022 and l catalyse similar site-specific integration and excision reactions between specific DNA regions known as attachment (att) sites. However, the Int proteins of HK022 and l are unable to catalyse recombination between non-cognate att sites.

The bacteriophage excisionase (Xis) is a sequence-specific DNA binding protein required for excisive recombination. Xis binds cooperatively to two DNA sites arranged as direct repeats on the phage DNA. Efficient excision is achieved through a cooperative interaction between Xis and the host-encoded factor for inversion stimulation as well as a cooperative interaction between Xis and integrase. The secondary structure of the Xis protein was predicted to contain a typical amphipathic helix that spans residues 18 to 28. Several mutants, defective in promoting excision in vivo, were isolated with mutations at positions encoding polar amino acids in the putative helix (T. E. Numrych, R. I. Gumport, and J. F. Gardner, EMBO J. 11:3797-3806, 1992). We substituted alanines for the polar amino acids in this region. Mutant proteins with substitutions for polar amino acids in the amino-terminal region of the putative helix exhibited decreased excision in vivo and were defective in DNA binding. In addition, an alanine substitution at glutamic acid 40 also resulted in altered DNA binding. This indicates that the hydrophilic face of the ␣-helix and the region containing glutamic acid 40 may form the DNA binding surfaces of the Xis protein.Site-specific recombination by bacteriophage is a complex process that requires the formation of nucleoprotein complexes composed of specific DNA sites in the phage and bacterial chromosomes and host-and phage-encoded proteins. Integrative recombination between specific attachment sites, attP on the phage DNA and attB on the bacterial chromosome, generates recombinant attR and attL sites flanking the prophage DNA (14). Excision of the prophage is accomplished by site-specific recombination between the attR and attL sites to regenerate the intact host and phage genomes. Both reactions are catalyzed by the phage-encoded protein integrase (Int), assisted by accessory proteins. The host-encoded integration host factor (IHF) is a protein required for both reactions. Excision requires an additional phage-encoded protein called excisionase (Xis). Excision is stimulated by the factor for inversion stimulation (FIS) supplied by the host. The directionality of recombination is determined by the amount of Xis present in the cell. During prophage induction, enough active Xis is present to promote excision (7,26). The amount of Xis protein present during establishment of lysogeny is limited due to instability of the protein (26). The integration reaction is inhibited by Xis in vitro (1, 17).Xis is a sequence-specific DNA binding protein of 72 amino acids (1). Xis recognizes two direct, imperfect 13-bp repeats, designated X1 and X2, on the attR site (see Fig. 1) (28). The FIS binding site, designated F, partly overlaps the X2 site (24,25). Xis binds DNA cooperatively at the X1 and X2 sites (4) and also binds to X1 cooperatively with FIS at the F site (19,24). Both Xis and FIS bend DNA upon binding to their specific sites (23). In addition, occupation of X1 by Xis facilitates binding of Int to the P2 site t...

Site-specific recombination by phages λ and P22 is carried out by multiprotein-DNA complexes. Integration host factor (IHF) facilitates λ site-specific recombination by inducing DNA bends necessary to form an active recombinogenic complex. Mutants lacking IHF are over 1,000-fold less proficient in supporting λ site-specific recombination than wild-type cells. Although the attPregion of P22 contains strong IHF binding sites, in vivo measurements of integration and excision frequencies showed that infecting P22 phages can perform site-specific recombination to its maximum efficiency in the absence of IHF. In addition, a plasmid integration assay showed that integrative recombination occurs equally well in wild-type and ihfA mutant cells. P22 integrative recombination is also efficient in Escherichia coli in the absence of functional IHF. These results suggest that nucleoprotein structures proficient for recombination can form in the absence of IHF or that another factor(s) can substitute for IHF in the formation of complexes.

Hypocalcemia can manifest as a variety of presentations, ranging from neuromuscular irritability to seizures, and psychiatric manifestations such as emotional instability, anxiety, and depression. Here, we present a unique case of hypocalcemia-induced acute psychosis.A 24-year-old woman presented to the emergency department (ED) with confusion and agitation for four to five days. The patient was noted by the family to have decreased oral intake and sleep. Auditory and visual hallucinations prompted the family to bring the patient to the ED. The patient was mildly tachycardic. Initially, the patient was agitated, impulsive, and aggressive, exhibiting psychotic features including visual hallucinations, paranoid delusions, thought broadcasting, tangential and perseverative thought processes, and erotomanic delusions. She had mild leukocytosis and elevated procalcitonin on admission. A thorough workup ruled out infectious/inflammatory processes. Cerebrospinal fluid was negative for acute meningitis/encephalitis, autoimmune encephalitis antibodies, and paraneoplastic etiology. Thyroidstimulating hormone was normal and thyroid antibodies were negative. The CT brain and MRI brain were unremarkable. The patient was severely hypocalcemic (6.7) with low parathyroid hormone (<6) on admission.To note, the patient has multiple endocrine neoplasia, type 2B (MEN2B). She underwent total thyroidectomy five months prior for metastatic medullary thyroid carcinoma complicated by postsurgical hypoparathyroidism. The patient had been non-compliant with calcium and calcitriol supplementation postoperatively.The patient was started on IV calcium gluconate and transitioned to calcitriol with calcium level improvement over the next three days. She experienced marked improvement, with the resolution of her psychosis. The patient's subacute onset psychosis with no personal or family psychiatric history and a rapid response to calcium correction supports hypocalcemia etiology. This case illustrates new-onset acute psychosis in a patient with calcium regulation imbalance. The development of hypocalcemia secondary to thyroidectomy with postsurgical hypoparathyroidism and calcium supplement non-compliance precipitated psychosis. A few similar cases have been reported, and here, we note that treatment of hypocalcemia promptly resolves symptoms. As per our review, this will be the first case of neuropsychiatric symptoms without associated cortical calcifications seen on imaging. It is important to recognize hypocalcemia as a rare cause of psychosis so as to not mistakenly categorize such presentations as primary psychotic disorders and miss a medically treatable illness.